CompTIA Network+ - Module 5 - Network Troubleshooting and Tools

Network Service Troubleshooting - Exhausted DHCP Scope

A DCHP server's pool has a certain number of available IP addresses. If you run out of addresses. You'll find the devices are not able to get an IP address from the DHCP server, and they'll assign themselves an APIPA address. If you find that devices are assigning themselves an APIPA address instead of assigning a DHCP address, you may want to check your DHCP server and that you have enough IP addresses available. And if possible, you may want to add additional IP addresses to the pool.

Hardware Tools - TDR and OTDR - Cost

A TDR/OTDR can be an expensive investment. These are thousands of dollars to have a piece of equipment that's able to perform these particular functions. And you're probably going to need additional training so that you know how to operate the equipment and understand the results that it's providing. But if you need some way to certify that your cables and your fibers are working as expected, this would be exactly the tool that you would use. You can validate everything about your installation and certify that all of your network connections will be working exactly to spec

Network Troubleshooting Methodology - Plan of Action - Document the Steps

A best practice is to document the exact steps that will be required to solve this particular problem. If it's replacing a cable, then the process will be relatively straightforward. But if you're upgrading software in a switch, a router, or a firewall, there may be additional tasks involved in performing this plan of action. You'll also want some alternatives if your plan doesn't go as designed. For example, you may run into problems when upgrading the software in a firewall. So you may need an additional firewall or way to roll back to the previous version.

Wired Troubleshooting - Bottlenecks

A bottleneck, in a communications context, is a point in the enterprise where the flow of data is impaired or stopped entirely. Effectively, there isn't enough data handling capacity to handle the current volume of traffic. A bottleneck can occur in the user network or storage fabric or within servers where there is excessive contention for internal server resources, such as CPU processing power, memory, or I/O (input/output). As a result, data flow slows down to the speed of the slowest point in the data path. This slow down affects application performance, especially for databases and other heavy transactional applications, and can even cause some applications to crash.

Wired Troubleshooting - Configurations Issues - Solutions - Duplex Mismatch

A more challenging problem to troubleshoot is when the duplex is mismatched between two devices. This is when one side is configured as half duplex and the other side is configured as full duplex. You'll have connectivity between the devices, but you'll notice the throughput is slower than you would expect. You'll also see an increase in the late collision counter on these devices, which might give you an indication that there is a duplex mismatch.

Hardware Tools - Spectrum Analyzer

A nice tool to have, especially for wireless networks, is a spectrum analyzer. This allows you to examine all of the different frequencies that happen to be in a particular range all at once, and you'll be able to tell exactly where most of the signal is in that particular frequency range. For example, if you're connecting a wireless access point for the first time, and you're wondering if there's anything else in the area that might be causing interference to slow down this network, you'll be able to see that with the spectrum analyzer.

Network Service Troubleshooting - Rogue DHCP Server

A rogue DHCP server, is a server installed by an outside party on your network with the capability of handing out ips to devices they choose as needed. And because there's no security inherent to DHCP, this might be something very easy for someone to configure and put on your network. This could mean that someone might be assigned an invalid or duplicate IP address. And that, of course, would affect many devices on the network and would probably prevent many clients from being able to communicate to other devices.

Wired Troubleshooting - Open and Short Circuits

A short circuit is two wires that are touching each other. So if someone has bent an ethernet cable, you may find that there is a short inside of that bend. An open is when the cable has been broken completely. There's no signal that's going to be able to make it through an open circuit. With an open circuit, there's obviously no communication that can occur across that open. But if it is a bent cable that's constantly moving, you may find there's intermittent connectivity with that short circuit.

Network Service Troubleshooting - ACL

Access control lists can provide extensive security options. You may find that they're blocking some traffic from getting through, but other traffic is able to flow properly. If you were to look at the access control list, you can see there are a number of different filtering options. You can filter by IP address, port number, and many other options as well. And you can allow or deny traffic based on a combination of this criteria.

Command Line Tools - ARP

Address Resolution Protocol: [arp] allows us to find the MAC address of a device when all we have is the IP address. You can see this ARP table in your Windows device by typing [arp-a].

Network Service Troubleshooting - Incorrect Host-based firewall

Apps can be filtered on your device with a host space firewall. A firewall administrator may be able to configure not just a port number, but the application name itself to be able to filter that traffic. In environments where the host-based firewall is administered centrally, you may not have access to view firewall information. So you may need to document exactly what application you need to use, and provide that information to the firewall administrator.

Wired Troubleshooting - Open and Short Circuits - Fixes

B/c opens and shorts may be inside the cable itself, it's very difficult to find exactly where that may be. You may just find that moving the cable a certain way causes the intermittent connection. These are also very difficult to repair. It's usually easier and less expensive to simply run a new cable and use that one instead of the one that has the short circuit. This is another example of where the TDR can help you find exactly the location where this particular open or short happens to be. By simply connecting to the wire, the TDR can tell you exactly how many feet away from the TDR this particular open or short happens to be.

Hardware Tools - Cable Crimpers

Cable Crimpers allow you to fasten the RJ45 connector to the end of the wire. Some crimpers also provide other connections so you can crimp RJ11 or coax connections on this same crimping device. This is really useful if you're running your own cable. You can cut cable exactly to the length you need, and then put your own connectors onto the end of those cables.

Network Service Troubleshooting - Date and Time

Configuring the date and time on all of the devices on your network become very important when you're trying to implement security. For example, the default tolerance for Kerberos is a five-minute window. So you have to have very tight tolerances on the time and date on all of your devices. This is because Kerberos is assigning you a ticket, and that ticket has a time stamp associated with it. If that time stamp is too old, Kerberos considers that ticket to be invalid and then your client is not able to log in. That's why one of the first things we do when there's a problem with Kerberos or being able to log in is to check the time stamp on the device that's trying to gain access to the network.

Wired Troubleshooting - Crosstalk

Crosstalk is one signal that's going across one pair of wires is affecting the signal on another pair of wires. This leaking of information from one wire to the other causes interference. And it may affect the overall performance of a connection.

Wired Troubleshooting - Decibels - Logarithms

Decibels are measured logarithmically. So if you were to measure twice the amount of signal across a line, you would say that it had increased 3 decibels. If you increase the signal 10 times, the difference is 10 dB. If you increase the signal by 100 times, then the difference is 20 dB. And if the signal increases by 1,000, you would say that the increase was 30 decibels.

Wireless Network Troubleshooting - Absorption - Materials

Different objects will absorb differently as the frequencies change. So you may find 2.4 GHz has a very different absorption rate than 5 GHz. This is one of the reasons we put wireless antennas on the ceiling. That way, the wireless signal can get to you without going through or being absorbed by other objects.

Wired Troubleshooting - EMI

Electromagentic Interference: Our cables are not indestructible, and we want to be sure that we're handling them properly and we're running them along areas where the EMI will be minimized. If you are running some new cables, you want to be sure not to twist them during the installation and minimize the amount of pulling or stretching that you would do to any set of cables. You also want to be sure that you don't have any sharp bends. Each cable will document the maximum bend radius allowed. You want to be sure not to extend over that bend radius. And of course, you don't want to use staples or any type of cable ties that might crimp the wires inside of those cables.

Wireless Network Troubleshooting - Wrong SSID

Every access point has at least one Service Set Identifier, or SSID, configured inside of that device. This designates the name of the wireless network that we would be connecting to. And if this is on your corporate network, it may be very obvious what SSID is associated with your particular network. But sometimes, this can be a challenge. You might bring up a list of available networks, and it may be the public WiFi internet, the Guest Internet, or the Internet SSID. Which one do you connect to? Some of them may be associated with your access points. Others may be associated with access points that are not part of your network. You may want to confirm what the correct SSID is and make sure that all of your users are connecting to the right wireless network.

Wired Troubleshooting - Transceiver Mismatch - Similar Models

Getting the wrong transceiver for a fiber connection is easy to do. Look at the trasnceivers shown, they have exactly the same format, the same connectors on the end. But if you look closely at these two transceivers, you'll notice one is designed for a 1,310-nanometer connection. And the other is designed for 850.

Wired Troubleshooting - VLAN Mismatch - Example

Here's the VLAN settings on my switch. I have everybody on this particular switch configured for VLAN 1. But if any of these interfaces showed on a different VLAN, I would need to check my documentation and make sure that's exactly the VLAN for that device.

Wireless Network Troubleshooting - Signal-to-Noise Ratio - Example

Here's the signal-to-noise ratio on my network. You can see the signal is designated by this green line on my wireless network, and the noise is the dotted lines. So you can see the noise is way down here at the bottom at negative 90 decibels, which is a small amount of noise, considering all of the other signal that's going through my network.

Wired Troubleshooting - T568A and T568B - Wires

Here's visually what these two standards look like. You can see the 568A has white/green, green, white/orange, blue, white/blue, orange, white/brown, and brown as numbers 1 through 8. The only differences between the A and B are on pins 1 and 2, and 3 and 6. Notice that pins 4 and 5, and 7 and 8, are exactly the same between these two standards.

Network Service Troubleshooting - Exhausted DHCP Scope - IPAM

IP Address Management: Exhausting a DHCP scope can sneak up on you, so you may want to implement some IP address management, or IPAM. This would allow you to monitor and get notification if your DHCP pool gets low.

Wireless Network Troubleshooting - Absorption

If a wireless signal isn't reflecting off of an object, then it may be absorbed by the object. That means the wireless signal is going to pass through an object and lose a little bit of signal as it passes through. This can be a problem as the wireless signal goes through walls or windows and it has less signal as it passes through on the other side.

Network Service Troubleshooting - IP Config Issues - Step 3 - Check surroundings

If the IP configuration on your device is not correct and not on your network or you don't have access to the documentation with the correct settings, you may want to look at other devices around you that seem to be working. You can look at their IP address, subnet mask, and default gateway, and see if that matches the subnet for your device. And of course, the problem may be associated with something else in your infrastructure. So you might want to perform some pings and traceroutes, and see just how far you're able to get outside of your local subnet.

Network Service Troubleshooting - IP Config Issues - Step 2 - Monitor Traffic

If the IP configuration on your device is not correct and you think your switch is configured with the wrong VLAN information and you're on the wrong IP subnet, you should be able to capture packets and at least see some information appear from your local subnet. That might give you some clues as to which subnet you're connected to.

Network Service Troubleshooting - IP Config Issues

If the IP configuration on your device is not correct, you may see a number of different symptoms occur. One of these might be that you can communicate to local IP addresses, but you're not able to communicate to IP addresses on a different subnet. Or you may find that there's no IP communication at all, and you can't communicate devices on your local subnet or a remote subnet. Or you may find that some IP addresses on your local subnet are accessible, but others are not accessible from your machine.

Network Service Troubleshooting - IP Config Issues - Step 1 - Documentation

If the IP configuration on your device is not correct. First check your documentation and make sure your computer's IP address, subnet mask, and default gateway matches what you show in your documentation.

Network Service Troubleshooting - Unresponsive Service - Hardware Failure

If the server that is hosting that application is having a problem, the issue may be very similar to the service itself not responding. We try to use the application, and we get no response from that device. We're going to first confirm the connectivity. So if we do have a hardware failure, we're probably not going to receive a response to this ping. We can also confirm that with a traceroute. So we can see exactly what hops we're going through to get to that server, but we can also see that the server is not responding to that traceroute. At that point, we want to check the server ourselves, or we'll need to contact the help desk or server administrator to see if they can find out why that server is not responding.

Network Service Troubleshooting - Duplicate IPs - Results

If two devices manage to connect to your network with the same IP address, you'll find that they'll fight with each other. One device will have connectivity and then the other device has connectivity, and it'll switch back and forth between the two devices. However, on most modern operating systems, the OS performs a check of that IP address before it connects to the network. And if it finds that IP address is already in use, it blocks your system from creating a duplicate.

Network Service Troubleshooting - Duplicate IPs - MAC Addresses

If you are manually configuring a device's IP address and you know it's the right address, but some other device is already using it, you can use a third device device to ping that IP address, find the MAC address of that device, and then locate that MAC address in your switch. That should tell you what interface that device is connected to.

Wireless Network Troubleshooting - Security Type Mismatch

If you do have a 802.1X being used for authentication to your wireless network and someone is not able to gain access, you may want to check the client configurations for their wireless device and make sure their wireless device is using a 802.1X as well. Since our wireless signals are going through the air, it's always import that we encrypt all of the communication we send over these wireless networks. And whenever we're connecting a client to the wireless network, the client's encryption must match what's set on the access point. This isn't usually a problem, because most of our devices these days are using WPA2 as the encryption type.

Network Service Troubleshooting - Duplicate MAC Addresses - Connectivity

If you do see multiple MAC addresses on your network, you may find that those devices have intermittent connectivity. The switch is going to be confused about exactly where that MAC address happens to be on the network. If you're trying to confirm the MAC address of a device, you may want to ping the IP address of that device and then look at your ARP cache to see exactly what MAC address is associated with that IP.

Network Service Troubleshooting - Exhausted DHCP Scope - Transient Users

If you have a lot of transient users that move in and out of the network every day, you might want to lower your lease time. This would allow more IP addresses to be released faster, and would provide a larger pool for other users that might need

Wireless Network Troubleshooting - Security Type Mismatch - Legacy Equipment

If you have any legacy equipment that does not recognize WPA2, those devices may have problems connecting to our modern wireless networks. Because of security issues associated with earlier encryption types, you may want to make sure that everyone is using at least WPA2 on both their devices and your access points.

Wired Troubleshooting - Configurations Issues

If you have problems with the configurations of the interfaces used on your network, you may see symptoms such as poor throughput. This would be constant poor throughput through any connection that you're using on your network. Or you may find some devices have no connectivity at all. You're not able to see any link lights appear on the ethernet adapter. Or there may be a link light but no activity light on the ethernet adapter. All of these situations might be easily resolved by checking the interface configuration of the ethernet adapters.

Wired Troubleshooting - Incorrect Cabling

If you look at the NIC statistics shows a lot of physical errors or CRC errors. You may want to check and make sure that you're using the right kind of cable. One way to tell is to simply look at the outside of the cable. And usually, there will be information printed onto the sheath itself. You might also want to get a TDR and run your own tests on this cable and make sure the specifications of what you're seeing on your TDR match what you're seeing on the outside of the cable.

Wired Troubleshooting - Transmit and Receive Reversal - Solutions - auto-MIDX

If you plug in a connection and you get no connectivity at all, you may want to look at the wire map and see if there was a transmit and receive reversal. If your ethernet adapter supports auto-MDIX, you may want to enable that and see if you're able to get a signal across the wire then. If you have identified a reversal, it's probably on the punchdown block or at the end device. So you may want to start with the patch panel and then work out from there to see if you can find where the reversal might be.

Wired Troubleshooting - Damaged Cables - Solutions

If you run into a cable that looks like it's been stepped on or bent, you may want to check the physical layer and make sure that the cable is working as expected. You might also want to look at the device the cable is connecting to. Because if that cable has been pulled, it might have bent pins inside of the ethernet adapter. Sometimes it's easy to simply replace a patch cable and see if that resolves the problem. But other times, you may need a TDR to see if there's a short, open, or some other kind of damage inside of that cable.

Network Service Troubleshooting - Duplicate IPs - DHCP

If you think you're getting duplicate IPs from a DHCP server, you may want to capture the packets associated with the DHCP process, and you'll be able to tell exactly which DHCP the server is providing you that duplicate IP address.

Network Service Troubleshooting - Untrsuted SSL Certificate

If you're communicating to a web server over an encrypted channel, and you receive a pop-up message in your browser, and this error says that the certificate is not trusted by your computer's operating system, then you may have a problem communicating securely to that web server.

Network Service Troubleshooting - Untrsuted SSL Certificate - Internal Certificates

If you're communicating to an internal web server on your company's network, then you may need to add your company's certificate authority to your browser. Normally this internal certificate is added by your workstation administration team, but you could manually add that certificate as well.

Wired Troubleshooting - Poor Performance

If you're finding poor performance on a device, you should look at the statistics associated with that NIC. And if you see a number of CRC errors or other types of errors on that connection, it may be related to a loss of signal. This is where it would be very useful to have a TDR or an OTDR. You can connect these advanced testing instruments to provide you with very detailed information of exactly how much signal you're able to put through a particular medium.

Network Service Troubleshooting - DNS Issues - Step 2 - nslookup or dig

If you're having DNS issues, and are able to ping your local device. You can then open a command prompt and use nslookup or dig to perform queries against that DNS server. You want to see if you're able to receive responses from the services that you would like to access. And if those DNS servers are not responding, you may want to try a different DNS server. Google's DNS servers are 8.8.8.8 and 8.8.4.4. Or you may want to try the servers at Quad9, which are 9.9.9.9.

Wired Troubleshooting - Cabling Issues - Interfaces

If you're having problems with the cable or fiber and you think there's an issue at the physical layer, you'll see errors appear on the interface that's connected to that cable. For example, you can look at your interface and see if there are any frame check sequence errors. You can see if there are any oversized packets or late collisions. And those might give you an idea that there is something happening with that physical layer.

Wired Troubleshooting - Pin-outs

If you're someone who crimps your own RJ45 connectors onto your ethernet cables, then you know it's very easy to switch cables around and have the incorrect pin-outs on these wires. When you plug in the wire, you may find that you're not able to go at the speeds you were expecting. Or there may be no communication at all across the wire. It also can be difficult to visually inspect the wire to see if you really did punch things down in the correct order. That's why it may be useful to get a cable tester like this one where you can simply plug in two sides of the tester to see exactly the pin-outs between one side and the other.

Network Service Troubleshooting - Unresponsive Service

If you're trying to communicate to a server and you're not getting any response, you know the problem isn't related to a filter or an ACL, and there may be a service that's simply not responding to your request. You may want to check and make sure that you're accessing that service over the correct UDP or TCP port number. And if it's different, you need to make that change in your application.

Network Service Troubleshooting - Unresponsive Service - ping and tracert

If you're trying to deal with an unresponsive service. You first want to confirm that the device itself is up and running. You may want to run a ping or a traceroute to the device, and make sure that you're able to communicate to that server successfully. And if you are, you might want to try telnetting to that particular port number itself and see if you're able to make the application talk back to you. If that application isn't responding, you may need to restart the application or restart the server where that application exists

Network Service Troubleshooting - ACL - Solutions

If you're trying to determine if an access control list may be blocking your traffic, you can perform a packet capture, be able to see exactly what traffic you're trying to send and what traffic is being received. You might also want to use a traceroute utility that allows you to customize the TCP or UDP port number. This would allow you to send traffic into the network, and you'd be able to tell at exactly which hop the traffic is stopping.

Network Service Troubleshooting - Expired IP Address

If you're using DHCP on your network, you know that most devices will be able to renew their IP address halfway through the lease time. If you find that the DHCP assigned IP address of a device is expiring, this may indicate a problem with the DHCP server. If a DHCP server is not available to renew that IP address, then the client will release that IP address at the end of the DHCP lease. We know that if an IP address then is starting with 169.254, then they have an automatic IP address assignment and they were not able to retrieve a DHCP assigned address. Your first place to go then would be your DHCP server. Make sure that you have addresses available in the pool and that the DHCP server is working normally.

Wired Troubleshooting - Transceiver Mismatch

If you're using transceivers in your network switches, you need to make sure those transceivers are matching the fiber or the connection that you're plugging in to them. For example, if this is a fiber transceiver, then the transceiver needs to match the wavelength of the fiber. So if you have 850-nanometer fiber, you need an 850-nanometer transceiver. You also want to check across the entire length that you're using the correct transceivers and the correct optic fiber. If you don't use the correct fiber or the correct transceivers, you'll see signal loss, dropped frames, missing frames, or other physical-layer problems.

Network Service Troubleshooting - DNS Issues

If your DNS isn't working, then you're going to have a difficult time resolving an IP address from a fully-qualified domain name. Web browsing and other applications aren't going to work, and it will seem as if the entire internet is down. You may want to try pinging IP addresses instead of a fully-qualified domain name to see if you're at least able to have connectivity to the network. And if you are able to ping these other devices by IP address, then you don't have a network connectivity issue.

Wireless Network Troubleshooting - Attenuation -Distance

In wireless networking, the closer you are to the access point, the less attenuation you're going to have. So you may find that moving closer to your access point greatly improves the performance of your network connection.

Wireless Network Troubleshooting - Predictable Interference

Interference on a wireless network means that something else is using the same frequencies that we're trying to use to communicate on this wireless network. Sometimes this is predictable interference. It may be a microwave oven or a fluorescent light, and we could easily turn off the light or turn off the microwave oven, and the interference will go away.

Wired Troubleshooting - Jitter - Bandwidth

It can be challenging to troubleshoot excessive jitter on your network. The first thing you might want to look at is how much bandwidth is available. If you're using an excessive amount of bandwidth on your internet connection, then it will be challenging to receive real-time information at regular intervals.

Hardware Tools - Loopback Plugs - Common Use

It's common to see loopback plugs used with serial connections or WAN connections. These could be RS-232 either 9 pin or 25 pin. They might be network connections for ethernet, maybe a T1 connection. There's even fiber loopbacks that you can use with fiber connections. It's important to note that this is not a crossover cable. Crossover cables connect like devices to each other. The loopback plug only has one single plug on it. You can see the wires come out of the plug and then back in to loop the signal back to itself.

Wired Troubleshooting - Jitter - Measurements

Jitter measurements are the time that we'll see between frames. So you should see a regular interval between each of the frames that are going across the network as time proceeds. If you start to have excessive jitter between these frames, then you'll have choppy phone calls and you'll lose frames on your video.

Wireless Network Troubleshooting - Attenuation

Just as we have attenuation on copper and fiber networks, we also have attenuation on wireless networks. As we move farther away from our access point, the signal gets weaker and weaker. And you can usually measure this by looking at the signal strengths on your device or using a WiFi analyzer.

Wireless Network Troubleshooting - Latency and Jitter

Latency can be a problem for wireless networks. The latency is the delay between transmitting information and receiving the response. We might also be concerned about jitter on our wireless networks. Jitter would be a deviation from a predictable data stream, very commonly associated with realtime communication, like voiceover IP.

Wired Troubleshooting - Latency

Latency is the difference between the request and the response. You can see a long latency may cause problems with your application. There's going to be a little bit of latency across the network because it takes time for the electrical signals to go down the wire and the electrical signals to come back the other direction. So there will be a bit of delay. It's when that delay becomes excessive that you start to have problems with your applications

Wireless Network Troubleshooting - Antennas - Incorrect Placement - Example

Let's say trying to find the best coverage for all of these rooms and all of the areas on this floor. So you might use a number of different access points with omnidirectional antennas. And you'll configure the access points to be on different non-overlapping channels. That way, no matter where you are on this particular floor, you have connectivity to the wireless network, and you can make sure that none of these channels are going to conflict with the others.

Hardware Tools - Light Meter - Mechanism

Light meters send the light from one side, which may be a laser or an LED, depending on the light meter and the type of fiber that we're using. And then we'll have a device that's going to measure how much of that light we're able to receive on the other side. If you have a very long fiber run and you're concerned that your equipment may not be able to see all of the light coming through, it's useful to use a light meter to be able to see exactly what the results will be once you connect your production equipment.

Wireless Network Troubleshooting - Frequency Mismatch - Differing Standards

Mixing different standards on the same wireless network may cause the network to not be as efficient as possible. For example, if you have an 802.11n network, and you've configured it to allow legacy 802.11b devices, you'll have additional information in the frame and additional frames that have to be sent, which will lower the overall efficiency of the network.

Wired Troubleshooting - T568A and T568B - Cosistency

Most people will 568A for horizontal cabling - that would be cabling that's on the same floor. But many organizations have used 568B. And as long as you've chosen one or the other and you stay consistent, you're not going to have a problem. If you do punch down one side of the cable with the 568A standard and the other side of the cable with 568B, you're not going to have a straight-through cable.

Wired Troubleshooting - Crosstalk - TDR - NEXT and FEXT

Near-End Crosstalk: One measurement a TDM provides when measuring crosstalk is NEXT. This is how much crosstalk is occurring as the signal is at the near end. It's the side that is transmitting that signal. You can also understand how much crosstalk you're seeing as it goes through the network on the other side by measuring at the destination. That's Far End Crosstalk, or FEXT.

Network Service Troubleshooting - Date and Time - NTP

Network Time Protocol: The easiest way to resolve date/time issues on your network is to configure all of your devices with the network time protocol, or NTP. This makes it so that every device can automatically update its clock and stay in sync with one another.

Wired Troubleshooting - Duplex and Speed Mismatch -Solutions

Often, we'll perform a bandwidth test to see exactly the throughput we're getting through that connection. And that might indicate that we have a duplex mismatch. You might also want to look at the ethernet statistics on that adapter and see if late collisions happens to be increasing. That may be another indication there is a duplex mismatch. So check the ethernet connections on both sides to see what both the speed and the duplex are set to.

Wireless Network Troubleshooting - Antennas - Types

Omnidirectional antenna like the one we see here would be put up on the ceiling, w/ an equal amount of signal that goes in every direction. This is good for a single building. But if we're trying to connect two buildings, though, we might want to use a directional antenna, where all of that wireless signal will be directed in one single direction, and you'll have the best chance of connecting those two buildings together. You might also find that your access point allows you to connect different types of antennas. So you may choose exactly the right antenna for the type of network that you happen to have.

Wired Troubleshooting - Attenuation

On any network connection, whether you're using fiber or copper, the signal will begin to degrade as it goes farther and farther in distance. We call this signal loss attenuation. And if we have too much attenuation on a wire or fiber, we won't be able to hear that signal when it reaches the other side.

Wired Troubleshooting - Attenuation - Conditions

On any of your networks, you'll have to consider attenuation when you begin engineering and troubleshooting these networks. If these are copper cables, you'll have electrical signals that will attenuate as they move through the cable. If it's a fiber connection, the same thing happens with light. And if these are radio waves on a wireless network, you'll have attenuation as you move farther away from the access point.

Wired Troubleshooting - Configurations Issues - Auto vs. Manual

On many networks, the ethernet cards are configured to automatically configure themselves when they connect to the network. They'll determine what's on the other end, and they'll make sure that the configurations match on both sides. But this doesn't work 100% of the time. So it may be better to set all of your connections up manually so they know that both sides are going to match.

Hardware Tools - Punch Down Tool - Best Practices - Maintaining Twists

On today's high speed ethernet networks, you also want to be sure to keep these twists in the wire- just as it shows here- as close as possible to where it's going into the punch-down block.

Wired Troubleshooting - Jitter

On today's networks, we have a lot of real-time applications. We're streaming real-time video. We have voice over IP communication. And all of this communication requires that there be a constant flow of traffic through the network. We expect these applications to send and receive traffic at regular intervals. If there happens to be delays between those intervals, you have jitter.

Network Service Troubleshooting - Blocked Ports

On today's networks, we're adding many different security devices. And we may find that certain application flows may be blocked due to filters installed on a firewall. This could also be configured as an ACL on a router, and it would be restricting the access for an application to travel through that network device. These security checkpoints are usually configured with very conservative rules, and it's not uncommon for these rules to block new applications from working on the network.

Wireless Network Troubleshooting - Signal-to-Noise Ratio

One good overall statistic to see just how much interference may be occurring on your wireless network is your signal-to-noise ratio. The signal is the normal communication that you want from your wireless network. The noise is interference that you might get from other devices and other wireless networks. And you would like to have a very large ratio between the signal and the noise. You would like to have much more signal than you're having of noise on your network. If you had as much signal as you had noise, it would be a one-to-one ratio, and it would be very difficult to communicate over that wireless network.

Wired Troubleshooting - Crosstalk - TDR

One good way to measure how much crosstalk you're having on a particular pair of wires is with a time-domain reflectometer. You may want to plug that in and get specific readings of crosstalk.

Wireless Network Troubleshooting - Overcapacity

One problem we really have with wireless networks is there's only so much capacity available. Only so many devices can be communicating over these very narrow frequency ranges that we have for wireless networks. So if you have too many devices on the same wireless network, you may run into problems with device saturation. If you have the option of using 5 GHz frequencies, you have many more frequencies available to use, and you may find that overcapacity isn't as big of a problem.

Wired Troubleshooting - No Signal vs. Intermittent

One symptoms associated with the dB loss is no signal at all, which means no connectivity from a particular device. In those particular cases it's relatively easy to troubleshoot because you know there's no signal coming through the line. But sometimes the problem is intermittent. Sometimes you have just enough to get the link up and running and maybe you can send some traffic across that network link. But you may find at other times that the network traffic is either not performing as you would expect or it's not able to communicate at all.

Network Service Troubleshooting - Duplicate MAC Addresses

One type of duplicate address you don't see very often is a duplicate MAC address. MAC addresses are burned into the network interface card. It's very unusual to see two interface cards with exactly the same MAC address. If you do see a duplication of MAC addresses, it could be something innocuous, like someone had misconfigured a manual MAC address configuration or the issue may be related to a locally-administered MAC address that has been misconfigured in a system, or sometimes you will run into a manufacturing error where two different interface cards have the same burned-in address.

Network Service Troubleshooting - Rogue DHCP Server - Switch Security

One way to disable this rogue DHCP server is to enable security on your switch. There's a function called DHCP snooping that may be able to identify rogue DHCP devices, and you may be able to authorize DHCP devices in Microsoft's Active Directory. And only those devices would be allowed to hand out DHCP addresses.

Wired Troubleshooting - Decibels

One way to numerically quantify the signal strength or loss of a signal is to use decibels (dB). In electronics and communications, the decibel is a logarithmic expression of the ratio between two signal power, voltage, or current levels. In acoustics, the decibel is used as an absolute indicator of sound power per unit area

Network Service Troubleshooting - DNS Issues - Step 1 - Checking local IP

Our applications commonly use fully-qualified domain names instead of IP addresses. So if you're not able to make that resolution, your applications are not going to be able to function. The first thing you may want to check is the IP address of your local device. If you're able to ping a device that's on another subnet, then you know you have the correct IP address, subnet mask, and default gateway. But you may want to check the IP configuration for your DNS servers. Make sure that IP addresses are listed under your DNS server configuration and make sure they are the right IP addresses for your DNS servers.

Wireless Network Troubleshooting - Antennas - Overlapping Chennels

Overlapping channels can be a challenge with 2.4 GHz networks, because there's only so many non-overlapping channels available. For example on this network, you can see someone is on channel 6 and another access point is on channel 11. And those two access points are not overlapping with each other. But then someone else may turn on another access point at channel 8, and now you have a frequency conflict with both channel 6 and channel 11, and now no one's network is going to be running at the best efficiency because of all the interference that's been introduced.

Wired Troubleshooting - Jitter - Real-time applications

Real-time applications are sensitive to any excessive jitter. That's because, if you're on a voice over IP phone call, you don't have time to have information retransmitted back to you. If you don't receive the information in time, then it's dropped and you can't ever rewind the conversation to begin again.

Wireless Network Troubleshooting - Reflection

Reflection is when a wireless signal bounces off objects. Some objects like desks or metal may bounce signals better than others, and the signal may bounce differently at 2.4 GHz than 5 GHz. If there's too much reflection, you may find that the signal is weaker than you might expect. But a little bit of reflection can actually be useful, especially if you are taking advantage of multipath interference with MIMO. This reflection may be something that you can work around by changing the direction of your antennas or where your antennas might be positioned. And again, reflection may not be as big of a problem if you're taking advantage of MIMO in 802.11n or multiuser MIMO in 802.11ac.

Command Line Tools - traceroute - Example - TTL

Regardless of OS, the basic operation of traceroute is going to be the same. Let's say that we're going to traceroute between Sam's workstation and Jack's workstation. We'll perform a traceroute to 10.10.30.10- that's Jack's workstation- over a maximum of 30 hops. That's the default for the traceroute utility. The number os hops is called the time-to-live value or TTL

Network Service Troubleshooting - Duplicate IPs

Some network administrators prefer to manually configure the IP addresses on all of their devices. They don't have a DHCP server, so they have to be very careful that they're not duplicating any IP addresses between devices. But of course, DHCP doesn't guarantee that you're not going to have duplicate IP addresses. You may find a combination of static IP addresses and an overlap with the DHCP pools, or you may have multiple DHCP servers, and you've accidentally configured duplicate IP addresses on both of those servers, or someone may turn on their own DHCP server without your knowledge, and now a rogue DHCP server is handing out IP addresses.

Wireless Network Troubleshooting - Unpredictable Interference

Sometimes interference is something that we can't predict. You may be in an office building with many other companies, and all of those companies have their own wireless networks, and you don't have a way to manage their access points so that there's no interference with yours. One way to look at interference statistics is to view them on your Linux or Mac OS 10 workstation with the netstat command. Or you may want to use Performance Monitor in Windows to monitor those statistics over time.

Wireless Network Troubleshooting - Wrong Passphrase - Shared Passphrase

Sometimes organizations use a shared passphrase. Everyone is given the same phrase, and they use that phrase to connect to the wireless network. This is very common in a SOHO setup, but not in an enterprise environment. In the enterprise environments, we prefer using technologies like a 802.1X. This is where you would configure your workstation to authenticate using your normal credentials. And those would be credentials that you would only know. Everyone would use their individual credentials, and that gives them access to the wireless network. This means there's no shared passphrase that you would pass around, and you could be sure that if somebody is gaining access to the network, that they used the correct credentials for your organization

Hardware Tools - Spectrum Analyzer - Wireless Spectrum

Spectrum analyzers view everything across a particular set of frequencies. So if the problem is a conflict with another wireless access point, you'll be able to see that in the spectrum analyzer. And if there are other devices in your environment that are causing interference on those same frequencies, it will also show up with the spectrum analyzer.

Software Tools - Speed Test Sites - Measuring Network Changes

Speed test sites can also be used if you're making any change to the network. You might want to run a speed test before you install a new firewall, and then run the speed test after you've installed the new firewall to see if there was any impact to the overall throughput. You might also want to try running speed tests at different times of the day. As the amount of utilization on the network changes, you'll be able to see the impact to the overall throughput to the internet.

Software Tools - Speed Test Sites

Speed test sites on the internet are a great resource if you need to quickly perform some bandwidth testing. These sites work by transferring a file and then determining how long it took for that file to be transferred. Many of them will perform download and upload tests, and they'll use different size files to be able to test different amounts of throughput.

Hardware Tools - TDR and OTDR - Logs and Reports

TDRs often work with software that allows you to log every single connection that you're testing, so at the end you can create a report that verifies that everything on the network is working as expected. Or if you're concerned that there's a cut in your fiber or copper, you can plug in a TDR and know exactly how far down that run you have the break.

Command Line Tools - tcpdump - options

Tcpdump has a number of different options that you can include at the command line. You can apply filters and you can watch the packets go by in real time. You can also choose to save all of the packets you've collected into a pcap-formatted file. This is a file format that's easily readable by many different protocol analyzers such as Wireshark.

Wired Troubleshooting - VLAN Mismatch - Solutions

The best way to check for a VLAN configuration is on the switch itself. So you would SSH or connect to the switch and see what the VLAN setting is for the interface that's connected to that device. VLAN 1 is usually the default for a switch. But many organizations will have many different VLANs. So you may need to check your documentation to see exactly what VLAN that device should be a member of.

Wired Troubleshooting - Damaged Cables

The copper cables that we use are pretty rugged. And when we run cables through a wall or over a ceiling, we don't usually have a problem with those cables. But very often, we have cables that are used for patches from the wall that are plugging into printers and other devices. And sometimes they can be stepped on. Or a cable can be pushed between a wall and a table. And then you have shorts and opens and problems communicating.

Network Service Troubleshooting - Rogue DHCP Server - Identifying the Rogue

The first thing to do with Rogue DHCP servers is to identify them. you would first have to identify the rogue DHCP server and disable it. You would then need to find all of the devices that received an IP address from that server, have them release that IP address, and then renew with the normal DHCP servers.

Wired Troubleshooting - T568A and T568B

The pin-outs that we use for our ethernet networks are T568A and T568B. They are 8 eight conductor 100-ohm balanced twisted-pair cabling. You may even see on different punch blocks and connectors that there may be different colors that will tell you, these colors are for A, and these colors are for B.

Wired Troubleshooting - Transmit and Receive Reversal - Solutions

The reversing of transmit and receive are easy problems to catch. You can easily see the wire map on a cable tester. And sometimes you can visually see it by examining the two ends of a cable or looking at the punchdown block itself. And some ethernet adapters will automatically adjust if they see that transmit and receive is reversed across a connection so that you're still able to communicate, even though the wire map isn't exactly the way it should be.

Wireless Network Troubleshooting - Antennas

The type of wireless antenna that you use will depend on the reach and the scope of your wireless network. If you're trying to cover wireless networks on a single floor of a building, you'll be using a different antenna than if you're trying to connect two buildings to each other.

Wired Troubleshooting - Bottlenecks - Solutions

There are many causes for bottlenecks, but sometimes it helps if you know what the normal type of communication should be. If you know that there shouldn't be any more than 100 milliseconds of difference of your database communication and you notice that suddenly you're getting 500 to 600 milliseconds of delay, then you know the problem is somewhere with that particular communication. And by resolving that bottleneck, you'll find that the overall performance of the application is improved.

Network Troubleshooting Methodology - Identify the Problem - Individual Process

There are times when you're examining a number of different problems that may not actually be related to each other. It's always best to separate all of these different issues out so that you can approach and try to resolve each issue individually.

Command Line Tools - DNS - command line options

There's a saying among network administrators that the problem is always somehow related to DNS. And because DNS is such an important part of our network communication, it's certainly important that you know how to perform troubleshooting with your DNS server. Two commands that can help you do this troubleshooting are [nslookup] and [dig]. Using either of these command line tools, we can query a DNS server and receive information back such as canonical name lookups, IP addresses, cache timers, and much more information that's contained on that DNS server. system.

Wired Troubleshooting - Crosstalk - Cabling - The Crimp

There's always going to be a little bit of crosstalk in a copper connection. But if you have excessive crosstalk values, you may want to look more at the cable. One good place to start is the crimp that you added to the end of the cable. You want to make sure that you maintain the twists as they're going into the RJ45 connector. And if there are other types of connections along the way, patch panels, for example, you want to be sure you're maintaining the twists on those as well.

Hardware Tools - TDR and OTDR

Time Domain Reflectometer: If you're working with copper cables, it's a TDR. If you're working with fiber, then it's an Optical Time Domain Reflectometer, or OTDR. These will be able to provide you with a lot of information about your copper cable or your fiber. For example, you can plug it into a connection, and it will estimate the entire length of the cable. Even if the cable's going into the ceiling and you're not able to see exactly how long it is, connecting the TDR will give you a very accurate representation of the length.

Network Service Troubleshooting - Blocked Ports - Solutions

To check a blocked app port, you can perform a packet capture. With this you can see the application request, and then you can see that no response is received. From there, you may want to run a traceroute tool that allows you to customize the TCP or UDP port number that's used. This would allow you to see just how far the traffic is able to go, and then you can provide that traceroute information to a network administrator who can then determine where the filtering is occurring.

Wireless Network Troubleshooting - Attenuation - Boosting

To help avoid excessive attenuation, you may be able to boost the signal strength on the access point itself. Not all access points have this option, but this may allow you to get a larger range out of your existing wireless access point. If the device you're on supports an external antenna, you may be able to use one with a higher gain. That will allow you to capture more of the signal, so even if there is attenuation, you're still able to receive and transmit on that network.

Command Line Tools - Nmap - device scanning

To scan for a device OSs. Nmap sends very specific queries to a device, and determines the OS from the response. Nmap is also able to determine the type of services and the versions of services that may be running on devices in a very similar way. It doesn't have to log in or use that service, it simply sends some very specific queries and examines the responses to those queries.

Network Service Troubleshooting - Duplicate IPs - Checking Devices

To troubleshoot duplicate IPs, you can start with the devices that are being manually configured. Check the IP address, subnet mask, and default gateway for your specific workstation, and make sure it matches your documentation. Another thing you can do is before bringing that station online, use a third station to be able to ping that IP address and see if another device responds. If another device does respond, you know that IP address should not be manually configured on another device.

Software Tools - Wireless Network Analysis - Challenges

Troubleshooting a wireless network can be a challenge. Obviously, wireless is going to be sending signal to whoever might be in the vicinity. Any device that wants to listen in to your network is able to do that because everything is going through the air. If you're going to use software that's listening in to this network, then that software needs to disable the transmission function of your wireless card because if you're transmitting, you won't be able to hear anything else on the wireless network. You also want to be sure you have the right kind of wireless interface card that's able to perform these analysis functions.

Command Line Tools - ARP - Example

Typing arp-a, shows the ARP cache entries that are available for both of the interfaces that are in this particular computer. There's a 10.1.10.17 that has an ARP cache. And I have another NIC on this device of 172.16.231.129, w/ its own separate ARP cache. Previously, my device has communicated to 10.1.10.1 and 10.1.10.2. And when it performed an ARP for those IP addresses, it received MAC addresses associated with those IPs and stored them in this ARP cache. If I need to communicate with those devices again, I don't need to perform another ARP across the network, I'll simply use what's stored in the cache.APC

Wireless Network Troubleshooting - Wrong Passphrase

Unless a wireless network is completely open to the public, you usually have to provide some type of authentication to gain access to a wireless network. And there are many different ways to provide that authentication. If you're finding that you're not connecting to the wireless network, it may be related to this authentication process. So you may want to make sure that you're using the correct credentials to gain access to the wireless network.

Wired Troubleshooting - Crosstalk - TDR - USing NEXT and FEXT

WIth NEXT and FEXT, you can look at the near end crosstalk to see how much crosstalk is introduced when the signal is at its strongest. And then you can look at the far end crosstalk to see how much crosstalk was introduced as the signal went through the cable.

Wireless Network Troubleshooting - Frequency Mismatch

We have a number of different 802.11 wireless standards, and some standards might use different frequencies than another standard. One thing that we have to make sure is that the devices that we're using are going to be matching the frequencies available on that access point. Sometimes you may run into problems where a client on the network is communicating to the access point, but over a slightly different frequency. You may want to check and see if someone may have manually configured a channel on their wireless device instead of having it set to the default, which is to automatically match what's on the access point.

Wired Troubleshooting - Duplex and Speed Mismatch

We know that for ethernet we can choose between 10 Mb, 100 Mb, or 1 Gb, usually on our devices. Or we could tell the device to automatically configure itself with the appropriate speed for that connection. You might also see options for duplex, such as half duplex, for full duplex, or again, you can tell the interface to automatically configure the duplex based on what it's connecting to. In many environments, the switch is configured to automatically negotiate the speed when a device is connected to it. But if there's a problem with the wiring of the cable or the end device is manually configured for 100 megabit, you may find that it's not the fast gigabit connection that you were expecting. And duplex, of course, is also automatically negotiated between the switch and the device that's connecting. And again, it needs to match on both sides of that connection. If there's a mismatch, there'll be a significant slowdown.

Wired Troubleshooting - Configurations Issues - Solutions - Speed

When checking for interface confirguration issues, he speed of the ethernet connection also needs to match on both sides. If you have 100 megabits on one side, it needs to be 100 megabits on the other. If there's a mismatch in those speeds, you'll have no connectivity across the network.

Wired Troubleshooting - Configurations Issues - Solutions - Link Lights

When checking for interface confirguration issues, the first thing you'll want to check for is a link light. That means you at least have connectivity between your device and the switch. If there's no light, then there is no connection. So there might be a cabling problem or an interface configuration issue.

Wired Troubleshooting - Crosstalk - Cabling - Shielding

When dealing with cable crostalk, you should consider using a different cable. You could use a shielded cable that is shielding between the different pairs. Or you might want to try a category 6A cable, which increases the cable diameter. And that means that you'll have a larger distance between those pairs.

Wired Troubleshooting - Latency Issues

When latency issues become prevalent, you want to use some measurement tools to be able to understand the exact latency that you're seeing for this application. It's very common to use packet captures and a protocol analyzer so that you can get very specific timestamps of when you send traffic across the network and how long it takes to receive a response to those requests.

Wireless Network Troubleshooting - Antennas - Incorrect Placement

When setting up antennas, you to make sure that you're putting these antennas in the right place. If you put access points too close to each other, you may find that those frequencies will interfere with each other. Or if the antennas are too far away from your users and there's other electrical devices causing interference, then you may find there's slower throughput than what's expected. Also check all of your access points to see if they're using the frequencies and the channels that you're expecting. You want to be sure you have the best coverage for your network, but you also want to be sure that you don't have any conflicts with any of those channels. F

Wired Troubleshooting - VLAN Mismatch

When you're configuring the interfaces on your switch, you're assigning each interface with a VLAN. And if you happen to put the wrong VLAN in for an interface, you may run into some problems. For example, a device may have a link light, which shows that it sees the switch on the other end, but it's not able to surf the internet or connect to any other devices. Or you may see an IP address that's assigned via DHCP, but it's not for the subnet that you thought you should be on. And if you configure the IP address manually, you still aren't able to connect to the devices on the network.

Wired Troubleshooting - Wiremap - Transmit and Receive Reversal

When you're running cable for a business, there are many different places where you might run into problems with the wire map. And it's very easy to reverse the transmit and the receives at the ends of the cable when you're putting on the RJ45 connectors. And you could also make a wiring mistake at the punchdown block itself.

Network Troubleshooting Methodology - Flowchart

When you're troubleshooting complex network problems, you may find that the resolution is not as obvious as you might hope. The methodology shown should help you troubleshoot any problem you run into when troubleshooting networks.

Hardware Tools - Punch Down Tool - Best Practices - Organization

When you're working with patch panels and punch-down tools, you want to make sure that you've got everything organized. You're going to have a lot of wires in one very small space. This patch panel helps by numbering each one of these connectors and showing you exactly where the wires go while you're punching them down.

Network Service Troubleshooting - Untrsuted SSL Certificate - Certificate Authority

When ytou get an untrusted SSL alert. This means that your browser received the certificate from the web server, but the certificate authority (CA) that signed that certificate is not in the browser's configuration. So the browser doesn't trust that certificate. This could be that the CA itself has not been signed by a certificate authority or the CA that has signed the certificate is not part of the trusted certificate authorities that are listed in your browser. You need to look at the certificate details itself. It will tell you what the issuing CA happens to be.

Wireless Network Troubleshooting - Latency and Jitter - Interference

Whenever you get on a wireless network, there is additional opportunity for interference and signal issues, because anything could be conflicting or interfering with that wireless signal. This would cause lower data rates, and it will cause retransmissions and loss of data. You might also run into challenges with latency and jitter if the network is very busy. If you're over capacity and have many different devices communicating, there may be slowdowns as more and more people join that wireless network.

Wireless Network Troubleshooting - Refraction

Wireless signals can suffer from refraction. This is when the signal is passing through an object and it exits that object at a slightly different angle. This might affect data rates, especially on connections where you're dealing with very directional signals. For example, outdoors between two buildings, where you have long distance links, and those links might have fog, water vapor, or temperature changes that might affect the refraction rates of those wireless signals.

Wireless Network Troubleshooting - Overcapacity - Bandwidth Saturation

With wireless networks you may run into problems with bandwidth saturation. This might be many people trying to transfer many files all at once, and you find that the wireless network simply doesn't have the bandwidth to support all of those transfers simultaneously. When we're managing our own wireless network on the floor of a building, we don't usually run into overcapacity problems. But when you get into very large environments, such as conference centers, airports, and hotels, you may often find the wireless network is not running as efficiently as you might have hoped.

Wired Troubleshooting - Cabling Nomenclature

You can see this one is rated for 550 megahertz, which automatically makes me think that this is at least a category 6 cable. And if we look at the outside, it does say that it's ETL well verified, TIA/EIA-568-B be standard. It is a UTP, or Unshielded Twisted-Pair cable. And it is a category 6-rated cable.

Wired Troubleshooting - Ethernet Adapter Configurations

You might also look at the ethernet adapter configurations on both sides of the connection and make sure the speed and the duplex match. If you're not communicating at all, you might also want to check the VLAN and make sure that both devices are configured to be on the same VLAN. And then it's very common to send traffic back and forth between those devices to see if any of these physical-level errors are going to increase as more traffic is sent over the connection.

Software Tools - Speed Test Sites - Using Multiple Sites

You might want to try different speed test sites, as well. Not every speed test site is built exactly the same. They might have a different number of servers, they might have more or less bandwidth, depending on what point of presence is being used, and the testing methodology may change depending on what service you're using.

Network Troubleshooting Methodology - Plan of Action - Impact on Network

You want to be sure that you're able to implement your plan of action with a minimum amount of impact to the production network, and sometimes you have to do this after hours when nobody else is working on the network. You want to be able to implement this with a minimum amount of impact to production traffic. So often, you'll have to do this after hours.

Wired Troubleshooting - EMI - Common causes

You'll find electromagnetic interference anywhere there's a power source. So if you're running your cables near electrical outlets or near fluorescent lights, you'll find there may be an excessive amount of EMI on your ethernet network. One good way to test for EMI is to use a TDR and see exactly how much signal and how much noise happens to appear on that link.

Wireless Network Troubleshooting - Antennas - Patch Antenna

You'll see it's designed to be mounted on a wall and has an interesting radiation pattern. It is exactly half of a sphere. So if I were to take a patch antenna put him up on a wall he would radiate all this direction but he wouldn't radiate behind him. So this could be a real benefit if you have it on a wall and you want to have everybody in the house to have signal but you don't want to project out onto the street. Patch antennas are extremely common in enterprise environments.

Wired Troubleshooting - Jitter - Device Causation

Your own switches and routers can contribute to this jitter. We want to be sure that none of those devices are queuing up information or have excessive congestion. And we want to be sure that those devices aren't dropping any frames as they're coming through the network. And in many environments, you're applying quality of service to the applications so that, if somebody is performing a very large file transfer, it won't affect any of your real-time communication.

Network Troubleshooting Methodology - Document Findings

After the problem has been resolved, this is a perfect time to document the entire process from the very beginning to the very end. You'll of course want to provide as much information as possible, so if somebody runs into this issue again, they can simply search your knowledgebase, find that particular error that popped up, and know exactly the process you used to solve this last time.

Software Tools - Network/Port Scanners

Another good software tool for any network administrator is a good port scanner. This will be able to scan IP addresses or a range of IP addresses, and find all of the open ports that may be available on those devices. More advanced port scanners can also tell you information about what operating system happens to be on that device and what type of services are running on that device.

Command Line Tools - route print

Another important piece of information when you're troubleshooting is knowing where traffic will be routed. In Windows, we can view this with the [route print] command.

Hardware Tools - Cable Testers - Limitations

Cable testers are relatively simple devices. These usually have just about eight lights on them. The one shown also has a ground light. And it will tell you if the light turns on then you have continuity, and if the light doesn't turn on, something's gone wrong with your crimping. If you need more advanced analysis of the cable, like crosstalk information or signal loss across the wire, then you'll probably want to bring in a TDR, or a Time Domain Reflectometer.

Hardware Tools - Cable Testers

Cable testers perform a continuity test across the wire, so you can very quickly see if pin 1 is connecting to pin 1, if pin 2 is connecting to pin 2, and so on. If something did go wrong, this cable tester can also tell you if pin 3 isn't connecting to anything on the other side, or if any wires may have been crossed between any of the pins.

Command Line Tools - traceroute - ICMP - Windows

Different OS use different methods to be able to perform a traceroute. For example, Windows sends an ICMP echo request and looks for the ICMP echo reply from the final device. This is commonly what Ping uses to send its messages. Of course, we're looking to receive an ICMP time to live exceeded message. And you also have to make sure that none of those protocols are going to be filtered between you and that final device.

Command Line Tools - DNS - dig

Domain Information Grouper: [dig] is effectively a replacement to nslookup for querying Domain Name System servers. The dig utility is not native to any of the Windows distributions, but you can download a version of dig for Windows from isc.org/downloads/bind.

Command Line Tools - tcpdump - example

Here's some data I captured using tcpdump. You can see it's a combination of IPv6 and IPv4 data. There's a lot of different kinds of data going back and forth. And this can be a little difficult to read if you're not familiar with exactly the format. This becomes a little bit easier over time. But you can see why it's useful sometimes to capture large amounts of data and then import that data later on into a protocol analyzer.

Command Line Tools - ipconfig - Example

If we were running Windows, we'd be able to see the allot of information such as the MAC, default gateway, and default gateway. As shown, this device was not able to get a DHCP address, so we can see an APIPA address on this device. And you can see if there are any IPv6 or other DNS server configurations all from the ipconfig command.

Software Tools - Protocol Analyzers

If you need to troubleshoot network slowdowns or application traffic flows, then you need to use a protocol analyzer. This will be able to capture every frame that's going back and forth between devices across the network, or capture traffic as it's going through your wireless networks. These protocol analyzers can be separate pieces of software that you would run on your computer, or this packet capture function may be built into the routers, switches, and other devices in your network.

Network Troubleshooting Methodology - Identify the Problem - User Input

Many times, network issues are identified by the end users, so they may be able to provide you with a lot more detail of what's really happening. Question your users to find out what they're seeing and if any error messages are appearing. It is important to know the change control process and exactly what is changing or has changed in your environment. Without some type of formal change control process, someone may be able to make an unscheduled change that would affect many different people. So when an error or network problem occurs, you may want to find out what was the last thing that changed on this network that could have affected all of these users.

Software Tools - Network/Port Scanners - Advanced Options

More advanced port scanners can do more than justscan IP addresses or a range of IP addresses, and find all of the open ports that may be available on those devices. They can allow you to pick a range of addresses to see who might respond to the scans that you're running, and some of these port scanners can build graphical representations of the results and now we can visually see exactly what the port scanners found.

Hardware Tools - Multimeter - Continuity

Multimeter allows you to perform continuity tests to see if you're getting connections between one end of a cable and another. If you don't have a cable tester, for example, you can use this multimeter to provide that continuity check against each pin of the cable, and build your own wire map using the multimeter.

Hardware Tools - Multimeter

Multimeters are tools that any one should have in their tool bag. They of course allow you to test the AC voltage so you can see if you are getting power from a power outlet. They also have options to be able to look at DC voltage as well, so you can check the voltage that's inside of the network devices that you're using.

Command Line Tools - Nmap - NSE

NMap Scripting Engine: Nmap includes its own scripting language that allows you to extend the capabilities of this very popular port scanner. This is NSE or the Nmap Scripting Engine. And there are a number of scripts that are included with Nmap that greatly extend its functionality.

Network Troubleshooting Methodology - Verify

Once you have executed on your plan of action, your job isn't done yet. We need to make sure that all of these changes actually resolved the problem. So now that the changes have been implemented, we now need to perform some tests. We may want to bring in the end users who first experienced this problem so that they can run through exactly the same scenario to tell you if the problem is resolved or if the problem still exists. This might also be a good time to implement some preventive measures. That way, we can either be informed that the problem is occurring, or we can provide alternatives that we can implement if that problem happens again.

Hardware Tools - Cable Snips and Wire Strippers

Once you start doing a lot of cable crimping, you'll know that you'll need to get not just a very good quality crimper, but you'll also need some electrician's scissors. These might be called cable snips. They're perfectly sized to be able to work on these very small cables. You might also want a good wire stripper, especially if you work with a lot of coax. A number of crimpers will also include a wire stripper on the crimper itself.

Hardware Tools - Punch Down Tool - Best Practices - Documentation

Once you're don punching down all your connections into the patch panel, You want to document exactly which one of these interfaces is connecting with which desk that's out on the floor.

Network Troubleshooting Methodology - Plan of Action

Once you've tested a theory and found that the theory is going to resolve this issue, you can then begin putting together a plan of action. This is how you would implement this fix into a production network.

Command Line Tools - ping - uses

Ping is usually the first thing you try to make sure that you can access a device across the network. And it's usually the last thing that you try after troubleshooting to make sure that the device is now available.

Command Line Tools - traceroute

Ping shows availability, and based on the time to live, the number of hops b/w our device and the destination device. By using the traceroute command [W:tracert, L:traceroute], we can map the route that's used between our device and that remote workstation. Traceroute is a utility that records the route (the specific gateway computers at each hop) through the Internet between your computer and a specified destination computer. It also calculates and displays the amount of time each hop took. Traceroute is a handy tool both for understanding where problems are in the Internet network and for getting a detailed sense of the Internet itself.

Software Tools - Network/Port Scanners - Rogue Systems

Port scanners are very good at finding and querying devices. So if you're concerned that there may be a rogue device on the network, these port scanners are very good at locating it. One of the most popular port scanners in the world is Nmap and the graphical Zenmap that works along with it, or you can also download other port scanners like Angry IP scanner, to find all of these different devices, operating systems, and services on your network.

Software Tools - Protocol Analyzers - Protocol Decodes

Protocol analyzers provide protocol decodes that allows us to see a frame-by-frame representation of exactly what's gone through the network. We can see the hexadecimal breakdown of the data within those frames, and we can get a protocol decode that gives us a plain English explanation of exactly what's inside of that frame. These protocol analyzers are not only good for providing information about network troubleshooting, but if you're storing large amounts of these packets, you can also find security issues that may be hiding in those frames.

Command Line Tools - route print - Example

Shown is [route print] on a Windows machine, and we'll get a breakdown of both IPv4 and IPv6 routing tables. At the top of the route print command, it tells us that there are four interfaces in this device. Two of them are physical network interfaces, one is the loopback interface, and the other is a tunneling interface used by Teredo. If we look at our IPv4 routing table, we can start to see the differences on where traffic might go. For example, anything that would go to a destination 10.1.10.0 with a netmask of 255.255.255.0, would go to 10.1.10.17 that has a metric of 281. But if traffic was being sent to 172.16.231.0, again with a 24-bit subnet mask, that traffic would be routed out 172.16.231.129. So by looking at these routing tables, we can tell where traffic may be going based on what that destination IP address is.

Command Line Tools - DNS - nslookup - example

Shown is a nslookup of www.professormesser.com. The response is that we are accessing a DNS server on Comcast.net. And there's the IPv6 address that I'm using to access that DNS server. And that server provides me with a non-authoritative answer that www.professormesser.com. is located at 104.20.215.41 and 104.20.214.1. In this case, there are multiple IP addresses configured in DNS so that if one IP address isn't available, the other is.

Command Line Tools - traceroute - Real World Example

Shown is a traceroute to 9.9.9.9. As we can see it will hop through all of the different routers on its way between my device and the 9.9.9.9 DNS server. You can see there were eight different hops to complete this route. We started with 10.1.10.1, which is my local router, and then all of the other routers in between until we finally hit dns.quad9.net. Now that we know the route that it takes to go between our workstation and Quad9, we can reference that later. If there's a problem between any of these particular links, we can run this traceroute again and see exactly where along the way we're having problems.

Command Line Tools - ifconfig - Example

Shown is ifconfig in MacOS, And I'm going to specify EN0 which is the ethernet adapter on my computer. And you can see, it does show me hardware information about this device along with the IPv6 and the IPv4 addresses that are defined on this computer.

Command Line Tools - pathping - example

Shown is pathping to 9.9.9.9, The first step will be a traceroute. We've seen this before when we ran the traceroute by itself. But after that finishes, you see that it will begin computing statistics for the next 200 seconds. Let's let pathping compute these statistics and see what the results are going to be. When pathping finishes its analysis, it will show you, again, the eight hops- the round trip time it takes to get to each hop. It will show you the amount of packets loss between the source device and this particular hop along the way. And then it also shows you how many frames were lost in between each one of these hops. Using pathping, you can start to see where problems may be occurring between different routers and be able to focus your troubleshooting efforts there.

Hardware Tools - OTDR - Radar Ping

The OTDR does exactly the same thing, except it's doing it with light. It sends the light down the fiber, and then it watches for any reflection to come back. Visually, we can see this happen as we send signals down the wire. Runs into a problem, and a little bit of that signal is reflected back to the TDR. Wherever we have that problem is where we're going to see the reflection come back to the TDR.

Hardware Tools - TDR - Radar Ping

The TDR is able to determine where breaks in a wire are because it's sending a ping of information down that wire, and it's listening for any reflections that are coming back from any problems. The TDR will then calculate the time that it took to send that signal from the time that it heard the reflection and tell you what the distance is between those two.

Command Line Tools - pathping

The [pathping] command utility in Windows combines [ping] and [tracert] together and adds some additional functionality. There are two phases to pathping. The first phase creates the path that runs a traceroute to build a map between you and another device. And in the second phase, it begins to measure round trip time on every link along the way. And you can see not only round trip time but also packet loss. This takes a little bit of time to run, but the results provide you with a nice view of how traffic may be flowing between each individual path along the way.

Network Troubleshooting Methodology - Identify the Problem - Information Gathering

The first step is to identify the problem. We first need to collect as much information as possible about the issue that's occurring. In the best possible scenario, you'll be able to duplicate this problem on demand. This will help later as we go through a number of testing phases to make sure that we are able to resolve this issue.

Hardware Tools - Tone Generator - Mechanism

Tone generators (and probes) make it very easy to find an individual wire, even when you're in a data center where there are hundreds of cables coming in to a single connection. You would first connect this tone generator to the wire. These usually have modular jacks and coax connections that can fit on to almost any type of cable. And then you go to the other end and start using the probe to try to find the wire that's connected to this tone generator.

Command Line Tools - traceroute - ICMP

Traceroute uses the ICMP (internet control message protocol) time to live exceeded message to be able to calculate what hop is the first hop, the second hop, the third hop, and so on. Your device not only needs to be able to receive these ICMP time to live exceeded messages, the routers that these packets are going through have to send you the ICMP time to live exceeded message. And not all routers will send that message back to you. Sometimes they are disabling that feature, or ICMP is filtered on that network.

Hardware Tools - Punch Down Tool - Mechanism

Using a punch down tool on a patch panel can be very tedious to do, because you have to put every individual wire into each connection on the punch-down block, and then manually punch down every single one of those. While you're punching this wire into the block, you would not only push the wire into the block, but also trim off any excess and keep that out of the way. So as long as you have all of your wires lined up, you can very quickly punch down all eight connections for a single ethernet drop.

Network Troubleshooting Methodology - Identify the Problem - Multiple Devices

When a problem happens on the network, it usually affects more than one device, and sometimes it affects those devices in different ways. You want to be sure to document all of the symptoms that may be occurring. Even if they are very different between different devices, you may find that a single problem is causing all of these different systems across these different devices.

Command Line Tools - tcpdump - data filtering

When capturing dat with tcpdump, the amount of information that you're gathering, and the detail included in these packets can be overwhelming. But if you spend a bit of time to learn the formats and be able to filter out what you don't need, you'll find there's a lot of valuable information stored in those packets.

Network Troubleshooting Methodology - Establish a Theory - OSI Model

When establishing a troubleshooting theory how to search is not always obvious. One method is to use the OSI model and start from the top with the way the application is working and work your way to the bottom. Or you may want to start with the bottom with the cabling and wiring in your infrastructure and work your way up from there. You'll want to list out every possible cause for this problem. Your list might start with the easy theories at the top, but of course include all of the more complex theories in this list as well.

Network Troubleshooting Methodology - Testing

When we have a list of theories on how to resolve an issue, we can now test those theories. We may want to go into a lab. And if we are able to recreate this problem in the lab, then we can apply each theory until we find the one that happens to resolve the issue. If you tried the first theory, you may want to reset everything and try the second theory or the third. And if you run out of theories, you may want to go back and think of other things that might be causing this problem.

Network Troubleshooting Methodology - Establish a Theory

When you've collected as much information as possible, you can examine all of these details to begin establishing a theory of what you think might be going wrong. Since the simpler explanation is often the most likely reason for the issue, that may be a good place to start. But of course, you'll want to consider every possible thing that might be causing this issue. Maybe start with things that aren't completely obvious.

Hardware Tools - Tone Generator - Inductive Probe

With a tone generator we don't have to break the wire open or touch any copper. This inductive probe allows us to do this without changing anything with the wire. This makes it very easy to find an individual wire, even when you're in a data center where there are hundreds of cables coming in to a single connection. You would first connect this tone generator to the wire.

Command Line Tools - ipconfig and ifconfig

You can determine the IP address of a Windows workstation using the [ipconfig] command. And on a Linux or OS X device, you would use the [ifconfig] command. These commands will tell you much more than just the IP address of this device. You'll get subnet mask information, the default gateway, DNS settings and any other configurations that are specific to the IP stack.

Command Line Tools - netstat

You can find netstat on Linux, on Unix, on Windows, and many others. [Netstat] stands for Network Statistics, and it provides you with many different views of what the statistics are for network communications on that particular device.

Command Line Tools - DNS - nslookup

[Nslookup] is an utility that can look up names and IP addresses from a DNS server. And although it's available on many different operating systems, the use of nslookup is now deprecated, and it's preferable that you use the dig utility instead.

Network Troubleshooting Methodology - Summary

- Identify the Problem - Establish a theory of probable cause - Test the theory to determine cause - Establish a plan of action - Implement the solution or escalate - Verify system functionality - Document Findings

Hardware Tools - Loopback Plugs

If you're trying to perform some troubleshooting on a network connection, you may be required to loop the signal coming out of that device back into the device that it came from. This is so it can perform some tests sending some signals to see exactly what type of signals it's receiving. The way we would this back is with a loopback plug. This allows us to test WAN connections, ethernet connections, and anything else that needs to send traffic out and loop it back to itself.

Command Line Tools - netstat -n

If you want to get rid of the resolved names and just get IP addresses with netstat, you can run netstat-n

Command Line Tools - netstat -a -b

If you want to see all of the active connections (ports) that are on an individual machine, run [netstat-a]. On a Windows machine, you could combine the netstat-a with the netstat-b [netstat -ab], which would not only show you the number of active connections, it would tell you exactly the Windows binary that was used to create that connection across the network.

Command Line Tools - iptables

If you're a Windows user, you're probably familiar with the Windows Firewall. If you're a Linux user, then you have many options for firewall in the Linux operating system, and one of the more popular versions is iptables. This allows you to provide stateful filtering in the kernel of the Linux operating system. It's common to see on workstations, servers, and any other device where you want to control the inbound and outbound network traffic.

Hardware Tools - Light Meter

If you're running your own cable and crimping RJ45 connections, it's very common to use a cable tester to make sure there's continuity for all eight of those pins. If you're running fiber connections, you may want to perform similar tests on the fiber. But on fiber, we're not checking for continuity. For fiber, we want to see just how much light is making its way from one side of the fiber to the other. we use a light meter that will be able to tell us exactly how much of our signal is making it through that fiber run

Network Troubleshooting Methodology - Testing - Consulting an Expert

If you're testing doesn't provide any results. You can bring in an expert who knows about the application or the infrastructure, and they can give some theories and possible resolutions to test in the lab.

Hardware Tools - Punch Down Tool

If you're working a lot with patch panels, then you'll probably be using a punch-down tool. This is the device that's going to push the cable into those sharp connections on the punch-down block and be able to lock those wires in place. You would use this with a 66 block or a 110 block most commonly on today's networks.

Hardware Tools - Tone Generator

If you've been working with a patch panel or with a large number of wires that's coming into a room, and you're trying to determine where the other end of a particular wire is, then you'll want to use a tone generator. This tone generator will put an analog sound on the wire, and then we use a separate inductive probe to find out where the other end of the wire happens to be.

Hardware Tools - TDR and OTDR - Splices and Impedance

If your cable has a cut or splice, you can plug in the TDR, and it will tell you exactly how far down the cable that particular problem exists. If you're simply trying to understand what type of cable it is, you want to understand the impedance on the cable, you can plug in the TDR and it will tell you that information as well.

Command Line Tools - tcpdump

In many network troubleshooting situations you're going to need a packet capture. Gathering packets from the network is going to provide you with a lot of detail of exactly what's happening across the network. One very common way to capture packets at the command line of a device is to use [tcpdump]. Tcpdump is usually included with many Linux distributions and OS X. And for Windows, you can download a Windows version of this called WinDump.

Software Tools - Wireless Network Analysis - Wireshark

In order to perform network analysis you would need a specific adapter card or wireless chipset to be able to put the card into this wireless analysis mode. But once you have the right adapter card, you're able to gather a lot of wireless-specific data. Not only IP traffic going across the wireless network, you can see the wireless protocols, the signal-to-noise ratio, channel information, and other wireless details. Using Wireshark on a wireless network can provide much of this information, and a number of third party tools can also gather these details from a wireless network.

Command Line Tools - traceroute - Example

In the example shown, Sam will send out a packet that has the traceroute TTL set to 1. That means when this packet hits the first router, the router will decrease the TTL by 1. And when the TTL = 0, which it will on this first router, a TTL exceeded is now sent back to Sam. Sam recognizes that that message came from Router 1 and says for this first connection, we received a TTL exceeded from 10.10.10.1 Now Sam's going to create another packet and this one, the time to live will be equal to 2. That means it will make it to Router 1 which will subtract one from the time to live number and send that packet on to the next route. This continues until finally in the last packet, where TTL = 4, it will make it through the first router, will make it through Router 3, will make it through Router 4, and finally, to Jack's workstation where the ICMP will be replied, and Sam will make a note of that device's IP address as the last route in the traceroute.

Command Line Tools - DNS - dig - example

Linux workstation, let's use the [dig] command to query www.professormesser.com. The format is a little bit different than nslookup, but the information is the same. Dig provides us with a breakdown of what our query happened to be. And in my case, I was questioning what the address is for www.professormesser.com. And you can see, the answer section shows me a little more information than nslookup did. It shows that www.professormesser.com has a time to live of 300 seconds before you would need to re-query that particular server. This allows me to change the IP addresses behind the scenes, and everyone will be caught up in 300 seconds. And it also shows me the two IP addresses. And then there's a summary of the query and exactly when the query was performed and how long it took.

Hardware Tools - Cable Crimpers - Cabling

Make sure that you're using the appropriate type of modular connectors for the type of cable that you have. The type of connectors that go on a category 5 are just a little bit different than those that go on a category 6 cable, so make sure you match your connectors with your cables. And although this can be a little frustrating at first, you very quickly can get the feel for pulling out all of these individual wires, putting them onto the connector, and crimping them down into the perfect sized cable.

Network Troubleshooting Methodology - Document Findings - Case Notes

Many organizations have a help desk with case notes that they can reference, or you might have a separate knowledge base or wiki that you create where you're storing all of this important information for the future.

Command Line Tools - Nmap

Network Mapper: It's a way to search for other devices and learn what services are running on your network. [Nmap] is probably best known as a port scanner. It will find open ports on a particular device, and tell you what those ports may be doing. It can also scan the device OS w/o having to log into the device. It's not native to the command line in windows but can be downloaded (or Zenmap as well)

Network Troubleshooting Methodology - Implement the Solution - Change Control Team

Once you've documented your plan of action, you can take that to your change control team, and they can give you a window when you can implement that change. The actual fixing of the issue is probably going to be during off hours during non-production times, and you may need to bring in other people to assist, especially if your window is very small.

Command Line Tools - ping

Ping [ping] is a way to test the reachability from your machine to another device somewhere on the network. Not only does it tell you if that device is responding to you, it also gives you an idea of the round-trip time between you and this other device. Ping uses ICMP or the Internet Control Message Protocol to be able to send these messages across the network.

Command Line Tools - Nmap - Basic scan example

Shown is a very simple Nmap scan. I'm going to choose one of the devices on my network. And it tells me very quickly that there were 991 ports that were closed. But it did show that port 22 was open, which is normally the SSH service. Port 80 was open for HTTP. Port 443 was open using HTTPS, and there's others here as well.

Command Line Tools - Nmap - OS Scan example

Shown is an Nmap OS scan [Nmap -0], And as it goes out and performs the scan, it will show me information about those open ports. But notice, it also shows me the MAC address of this device which is a Synology device. And it is indeed running Linux. And it was able to tell me that all based from this Nmap scan.

Command Line Tools - ping - Mechanism

Shown is ping in action, in the command line you'll type ping and then we'll choose an address to ping (ex 9.9.9.9). Windows will send four separate ping attempts, and it will tell you if you received a reply for each of those attempts. For example, we received a reply from 9.9.9.9. 32 bytes was the size of the ping that I sent, and it was the size that I had returned to me. It took 14 milliseconds. And the time to live for this TCP packet is 57 hops. And you can see I performed four separate pings with this single command, and I have the round-trip times that are pretty close to each other- 14 milliseconds, 18 milliseconds, 17 milliseconds and 13 milliseconds. Overall, I sent four packets and received four packets for a 0% loss to 9.9.9.9

Command Line Tools - netstat - Example

Shown is the netstat commnand run with no other parameters, the result shows me all of the network communications that I have made. You can see a number of them out to professormesser.com and to other web services. And as it's working through all of the active connections, it begins updating those on the screen.

Hardware Tools - Cable Crimpers - A "good" crimp

Shown is the result of an RJ-45 crimp, All of the cables are through the entire part of the connector, and you can see that those sharp metal pieces have now been pushed into the individual wires, all the way through these eight different connections on the RJ45 connector. You also notice on the back of the connector is a cable stay that has also been pushed into hold this entire cable in place.

Command Line Tools - netstat -o

Shows the executable and process id for every connection. You find the corresponding application by going into task manager and checking the ID in the PID column in the Details tab

Command Line Tools - netstat -r

Shows you the routing table for you current machine

Command Line Tools - traceroute - ICMP - Linux and IOS

Some operating systems give you more control over the traceroute. Linux, Unix, and Mac OS allow you to change what protocol you'd like to use during the traceroute. And if you're using a mobile device that runs iOS, it's going to send UDP datagrams over port 33434. You may be able to change this port number with some of the extended options available in iOS.

Command Line Tools - iptables - options

Some organizations will use iptables as the primary firewall between the inside of their network and the internet. And they can configure iptables to filter by IP address, port, application, and other criteria. Some Linux distributions use firewalld instead of using iptables, but there's many options available for controlling network traffic on a Linux device.

Software Tools - Speed Test Sites - Site Examples

Some very common third party test sites are SpeedOf.Me, speedtest.net, and testmy.net. If you're on an ISP, they may ask you to perform speed tests on their local network. So if you're on Comcast, you may be using speedtest.comcast.net, and AT&T has att.com/speedtest.

Hardware Tools - Cable Crimpers - Modular Connectors

The crimper works by pushing the sharp metal prongs that are on the inside of the RJ45 connector into the insulation that's around each individual wire. The crimper is also pushing a connection in on the back of the RJ45 connector that fastens it securely to the outside of the cable sheath. Shown are the RJ45 modular connectors before they're crimped. You can see they're sticking out just a little bit with these copper connectors. And look at those sharp metal pieces that are on the end of the connector. That's what's going to push into the insulation that's around each individual wire.

Hardware Tools - TDR and OTDR - Infrastructure Use

We commonly use these TDRs and the OTDRs when we're first installing a cable infrastructure, b/c you can plug these devices in and see just how much signal we're losing between one side of the cable and the other. This is especially good for fiber, where you want to be sure over very long distances that you're minimizing the amount of light loss over that run.