Troubleshooting Networks
How do you fix a problem causing you to experience IP conflict?
IP address conflicts are somewhat rare when DHCP is in use, but they can still happen. DHCP servers and clients both check for IP duplication when the DHCP client receives an IP address, but the process doesn't always work. Moreover, if someone with a statically configured IP address connects to the network with the same address as another machine, a conflict will exist. Using the ipconfig/release and ipconfig/renew commands will usu-ally solve the issue. You can determine the MAC address of the computer with which you are experiencing the conflict by using the ping command followed by the arp -d command.
What is a APIPA?
Automatic Private IP Addressing (APIPA) is a TCP/IP feature Microsoft added to its operating systems. If a Dynamic Host Configuration Protocol (DHCP) server cannot be found, the clients automatically assign themselves an IP address, somewhat randomly, in the 169.254.x.x range with a subnet mask of 255.255.0.0. This allows them to communicate with other hosts that have similarly configured themselves, but they will be unable to connect to the Internet or to any machines or resources that have DHCP-issued IP addresses.
How do you fix a problem causing you to experience low RF signal?
In a wireless network, the signal coming from the AP has a distance limit. With some variation by standard, this is about 100 meters. However, this distance is impacted by obstructions and interference in the area. The WLAN design should include a site survey that identifies these issues and locates APs and antenna types in such a way as to mitigate these effects. It is also useful to know that APs and some client radios have a setting to control signal strength. It is not a normal practice to change the setting in a laptop wireless card, but it may be necessary to change the transmit level on an AP. In many cases, it is actually beneficial to reduce the transmit level of an AP in situations where it is interfering with other APs in your network or you want to limit the range of the signal to prevent it from leaving the building. This is especially true in high-density areas where several APs are co-located in the same area for increased throughput.
How do you fix a problem causing you to experience not being able to find the SSID??
In an 802.11 WLAN, the service set identifier (SSID) is used as both a network name and in some cases the magic word that allows access to the network. One of the ways you can increase the security of a WLAN (not sufficient in and of itself but a good addition to a lay-ered approach to WLAN security) is to "hide" the SSID. This is also referred to as disabling SSID broadcast. It is accomplished by setting the AP to not list the SSID in the beacon frames. These frames contain the information that is used to populate the list of available wireless networks when you "scan" for wireless networks on your wireless device. When the SSID is hidden, the only way a device can connect to the WLAN is to be configured with a profile that includes the SSID of the WLAN.
How do you fix a problem causing you to experience limited connectivity?
In some cases, the computer has connectivity to some but not all resources. When this is the case, issues that may reside on other layers of the OSI model should come under consideration. These include the following: Authentication Issues: Does the user have the permission to access the resource? DNS Issues: You may be able to ping the entire network using IP addresses, but most access is done by name, not IP address. If you can't ping resources by name, DNS is not functional, meaning either the DNS server is down or the local machine is not configured with the correct IP address of the DNS server. If recent changes have occurred in the DNS mappings or if your connection to the destination device has recently failed because of a temporary network issue that has been solved, you may need to clear the local DNS cache using the ipconfig /flushdns command. Remote Problem: Don't forget that establishing a connection is a two-way street, and if the remote device has an issue, communication cannot occur. Always check the remote device as well. Any interconnecting device between the computer and resource, such as a switch or router, should also be checked for functionality.
How do you fix a problem causing you to experience slow transfer speeds?
Slow transmission on the network can be caused by hardware and software issues. Some of the physical issues that can cause slow performance are as follows: Interference: Both wireless and wired networks can be affected by electromagnetic inter-ference (EMI) and radio frequency interference (RFI). EMI will degrade network perfor-mance. This can be identified by the poor operation you may experience. Be sure to run cables around (not over) ballasts and other items that can cause EMI. RFI is a similar issue introduced by radio waves. Wireless networks suffer even more from both of these issues. Incorrect Cabling: The network can go only as fast as its weakest link. Using CAT3 cabling, for example, will only allow the network to operate at 10 Mbps even if all the net-work cards are capable of 10 Gbps. Malfunctioning NIC: NICs can malfunction and cause a broadcast storm. These broad-cast packets fill the network with traffic that slows performance for all users. Use a proto-col analyzer to determine the MAC address of the offending computer. From a software standpoint, the following issues can result in less than ideal performance: Router Misconfiguration: If the router is not configured correctly, it can cause slow performance because of less than optimal routing paths. Escalate the issue to the appropriate administrators. Switch Misconfiguration: An improperly implemented redundant switch network can result in switching loops that cause slow performance. Escalate the issue to the appropriate administrators.
What do you do if your trying to establish connection between a PC and a network and the network uses DHCP for IP configuration and the computer with the connectivity issue has an APIPA address?
The problem is one of these three things: ■The DHCP server is out of IP addresses. ■The DHCP server is on the other side of a router and there is no functional DHCP relay present or no IP helper address configured on the router—all of which is to say the DHCP request is not reaching the DHCP server. ■The computer has a basic connectivity issue preventing it from connecting to the network.
How do you fix a problem causing you to experience intermittent connectivity?
When a connectivity issue comes and goes, it can be a hardware issue or a software issue. The following hardware components should be checked for functionality: Network Cable: A damaged cable can cause intermittent connectivity. Network Interface Card: If the NIC is not properly seated or has worked its way partially out of its slot, it can cause connections that come and go. Interference: On a wireless network, cordless phones, microwave ovens, and other wireless networks can interfere with transmissions. Also, users who stray too far from the AP can experience a signal that comes and goes. The following are software issues that can cause intermittent connectivity: DHCP Issues: When the DHCP server is down or out of IP addresses, the problem will not manifest itself to those users who already have an IP address until their lease expires and they need a new address. In this case, some users will be fine and others will not, and then users who were fine earlier in the day may have problems later when their IP address lease expires. DNS Problems: If the DNS server is down or malfunctioning, it will cause problems for DNS clients who need name resolution requests answered. For users who have already connected to resources in the last hour before the outage, connectivity to those resources will still be possible until the name to IP address mapping is removed from the client DNS resolver cache.
How do you fix a problem causing you to experience no connectivity?
When no connectivity can be established with the network, your troubleshooting approach should begin at the physical layer and then proceed up the OSI model. As components at each layer are eliminated as the source of the problem, proceed to the next higher layer. A simple yet effective set of steps might be as follows: 1. When dealing with a wireless network, ensure that the wireless card is functional. The wireless card is easily disabled with a keystroke on a laptop and should be the first thing to check. If the network uses a hidden SSID, ensure that the station in question is configured with the correct SSID 2. Check the network cable to ensure it is the correct cable type (crossover or straight through) and that it is functional. If in doubt, try a different cable. 3. Ensure that the NIC is functional and TCP/IP is installed and functional by pinging the loopback address 127.0.0.1. If required, install or reinstall TCP/IP and/or replace or repair the NIC. 4. Check the local IP configuration and ensure that the IP address, subnet mask, and gateway are correct. If the default gateway can be pinged, the computer is configured correctly for its local network, and the problem lies beyond the router or with the destination device. If pings to the gateway are unsuccessful, ensure that the IP configurations of the router interface and the computer are compatible and in the same subnet.
How do you fix a problem causing you to experience unavailable resources?
When resources are unavailable, keep in mind that the issue could lie either in the source device or in the destination device that is holding the resource. It also might be a router or switch between the devices. Start your troubleshooting at one end or the other and using the same method used when experiencing no connectivity. Check the connections between each pair of intermediate devices until your locate the problem devices. Internet: When all other access is fine but there is no Internet access, the issue is frequently DNS. Local resources: When a computer can communicate only on its local network or subnet, the problem is usually one of the following: Incorrect Subnet Mask: Sometimes an incorrect mask will prevent all communication, but in some cases it results in successful connections locally but not remotely (outside the local subnet). The subnet mask value should be the same mask used on the router interface connecting to the local network. Incorrect Default Gateway Address: If the computer cannot connect to the default gate-way, it will be confined to communicating with devices on the local network. This IP address should be that of the router interface connecting to the local network. Router Problem: If all users on the network are having connectivity problems, you likely have a routing issue that should be escalated to the proper administrators. Shares: When troubleshooting issues regarding shares, keep in mind that the issue could be a network connectivity problem, in which case you would use the methods described in the sections "Limited connectivity" and "No connectivity" to isolate and resolve the issue. However, the issue could be permissions. It could be that either the user lacks the proper permissions or the permissions are misconfigured. You should check first with the data owner (typically a department head but not always) to verify the proper permissions that should be in place and, if required, configure them correctly. You may have to inform the user that they currently lack said permission and direct them to the data owner for the permission needed. Never take it on yourself to make that decision. Email: Email connectivity issues follow the same troubleshooting approach covered in the previous section "Limited connectivity." As discussed in that section, email issues typically revolve around DNS issues, network connectivity, or a failure in authentication. One issue to check is the setup of the account. For example if the client is set to use POP3 and the server is using IMAP, there will be a connectivity issue.
What are the steps to connect to a WLAN when the SSID is hidden?
When the SSID is hidden, the only way a device can connect to the WLAN is to be con-figured with a profile that includes the SSID of the WLAN. While every operating system is slightly different, to do this in Windows 10, you follow these steps: 1. Open the Network And Sharing Center. 2. Select Set Up A New Connection Or Network. 3. Select the option Manually Connect To A Wireless Network and click Next. 4. Complete the network name, security type, encryption type, and security key; check the box Connect Even If The Network Is Not Broadcasting; and click Next. Now the profile is complete and you should be able to connect to the "hidden" network. To make it easier for the user, you may also want to check the box Start This Connection Automatically.