CTC 228 Midterm
Basic Switch Operation
1. The switch receives a frame. 2. The switch reads the source and destination MAC addresses. Computer A IP address: 10.1.1.1 MAC address: AA:A1 3. The switch looks up the destination MAC address in its switching table. 4. The switch forwards the frame to the port where the computer owning the MAC address is found. 5. The switching table is updated with the source MAC address and port information.
Physical Bus Limitations
30 computers 185 meters of cabling Can be made longer by added repeater
Ethernet Frames
A frame is the unit of network information NICs and switches work with. It's the NIC's responsibility to transmit and receive frames and a switch's responsibility to forward frames out the correct switch port to get the frame to its destination. They come in 4 different formats or frames
Hub
A hub is a common connection point for devices in a network. Hubs can only transmit data at one computer at a time
Mesh Topology
A mesh topology connects each device to every other device in a network. You can look at a mesh topology as multiple point-to-point connections for the purposes of redundancy and fault tolerance. Each switch is connected to every other switch, which is called a "full mesh." If each switch were connected to only two other switches, it would be called a "partial mesh." In either case, the purpose of creating a mesh topology is to ensure that if one or more connections fail, there's another path for reaching all devices on the network. This type of topology is used mostly commonly in large internetworks and WANs, where routers or switches in multiple buildings or towns are connected in a partial or full mesh. Parts of the Internet are also designed with a partial mesh topology, in which major ISPs are connected so that even if one ISP's network fails, data can bypass this part of the network to get to its destination. Mesh topologies, although reliable, are also expensive because of the additional cabling and ports required. In most cases, the ports used to connect devices are the highest speed available, such as 1 Gbps or 10 Gbps, and they often use expensive fiber-optic cabling for connecting buildings.
Input
A user running a word-processing program types the letter A on the keyboard, which results in sending a code representing the letter A to the computer.
Address Resolution Protocol
Address Resolution Protocol (ARP) is used to resolve a logical (IP) address to a physical (MAC) address. When a system begins a conversation with a host and doesn't have its MAC address to create the frame header, it sends an ARP broadcast frame requesting the MAC address corresponding to the host's IP address. A net- work device configured with the specified IP address responds with an ARP reply message containing its MAC address. Then the packet is sent to the Network access layer, and the frame can be constructed.
Subnetting with IPv6
Although subnetting as done in IPv4 will be a thing of the past, it doesn't mean subnetting won't be used at all in IPv6 networks. Typically, ISPs allocated IPv4 addresses to businesses in groups specified by a network address and an IP prefix. ISPs try to give a business only the number of addresses it requires. However, with IPv6 having such a large address space, most address allocations will have a /48 prefix, even for small home networks. This means the network ID is 48 bits, and the network administrator has 80 bits for assigning subnets and host IDs. Because the host ID is 64 bits, 16 bits are left for creating subnets. This number of bits allows 65,536 subnets, more than enough for all but the largest organizations. Large conglomerates can get multiple /48 prefix addresses or /47 prefix addresses, whichq provide more than 130,000 subnets. A typical IPv6 address, then, as assigned by an ISP looks like Figure 5-15.
Entrance Facilities
An entrance facility is the location of the cabling and equipment that connects a corporate network to a third-party telecommunications provider. It can also serve as an equipment room and the main cross-connect for all backbone cabling. This location is also where a connection to a WAN is made and the point where corporate LAN equipment ends and a third-party provider's equipment and cabling begins—also known as the "demarcation point."
Point to Point Topology
As its name implies, a point-to-point topology is a direct link between two devices. It's most often used in WANs, in which a device on a business's network has a dedicated link to a telecommunication provider, such as the local phone company. The connection then hooks into the phone company's network to provide Internet access or a WAN or MAN link to a branch office. The advantage of this type of topology is that data travels on a dedicated link, and its bandwidth isn't shared with other networks. The disadvantage is that this topology tends to be quite expensive, particularly when used as a WAN link to a distant branch office. Point-to-point topologies are also used with wireless networks in what's called a wireless bridge. This setup can be used to connect two buildings without using a wired network (see Figure 3-5) or to extend an existing wireless network.
Logical Topologies
As mentioned, a network's logical topology describes how data travels from computer to computer. In some cases, as with a physical bus and physical ring, the logical topology mimics the physical arrangement of cables. In other cases, as with a physical star, the electronics in the central device determine the logical topology. A network's logical topology reflects the underlying network technology
IP Addressing
As you've learned, IP is responsible for addressing and routing in the TCP/IP environment. IP addresses are 32-bit (4-byte) logical addresses. The 32 bits are grouped into four 8-bit octets, and each octet is represented by a decimal number from 0 to 255. The four decimal numbers are separated by periods in a format called dotted decimal notation, as in 172.24.208.192. As discussed, an IP address is divided into two distinct parts. One part designates which logi- cal network the computer is a part of; the remainder of the address represents the host ID for that computer. For example, a computer with the address 172.24.208.192 resides on the 172.24 network, and its host ID is 208.192. In this case, the complete network address is expressed as 172.24.0.0, with the trailing zeros indicating a network address because a host ID can't be 0. The computer next to it might have the address 172.24.18.26, but even though their host IDs are quite different, both computers are on the same network because they share the same network address (172.24). You can determine how many host addresses are in a network (the address space) by looking at the host ID's size. In the address 172.24.208.192, for example, the host ID occupies the third and fourth octets of the address, which allows 16 bits for the address space. With 16 bits of address space, you can use the formula 216, which yields 65,536. An address with a network number of 201.55.66 leaves only one octet (or 8 bits) for the host ID, or 28, which yields 256 possible addresses.
NICs and MAC Address
Aside from the tasks described previously, a NIC has the important function of giving a computer a MAC address, an integral part of each NIC. NIC manufacturers ensure that every NIC has a unique address because networks won't function correctly if duplicate MAC addresses exist. The MAC address is stored in read-only memory (ROM) on the NIC. Because the address is said to be burned into memory, it's sometimes referred to as the burned-in address (BIA). The MAC address is composed of two 24-bit numbers: ● A 24-bit manufacturer ID called an organizationally unique identifier (OUI) ● A 24-bit serial number assigned by the manufacturer The 48-bit MAC address is expressed in hexadecimal notation, usually as six two-digit alphanumeric characters separated by dashes or colons, such as 04-40-31-5B-1A-C4. The first three two-digit groups represent the OUI, and the last three are the unique serial number.
Bandwidth Rating
Bandwidth, the number of bits per second that can be transmitted across a medium, is as much a function of the technology used to transmit bit signals as it is of the medium. For example, Category 5 UTP cabling was originally intended to support only up to 100 Mbps but was later upgraded to support up to 1000 Mbps when the 1000BaseT standard was developed.
Private IP Address
Because of the popularity of TCP/IP and the Internet, unique IP addresses to assign to Internet-accessible devices are almost exhausted. To help alleviate this problem, TCP/IP's technical governing body reserved a series of addresses for private networks—that is, net- works whose hosts can't be accessed directly through the Internet. This nonprofit governing body, the Internet Engineering Task Force (IETF; www.ietf.org), is responsible for TCP/IP standards and characteristics. The reserved addresses are as follows: ● Class A addresses beginning with 10 (one Class A network address) ● Class B addresses from 172.16 to 172.31 (16 Class B network addresses) ● Class C addresses from 192.168.0 to 192.168.255 (256 Class C network addresses) The addresses in these ranges can't be routed across the Internet, which is why any organiza- tion can use them to assign IP addresses to their internal hosts. If access to the Internet is necessary, a process called Network Address Translation (NAT) is used, explained next. IPv6 eliminates the need for private addressing because it provides a 128-bit address space, compared with IPv4's mere 32 bits. You learn more about IPv6 later in this chapter in "Introduction to Internet Protocol Version 6."
Cable Grade
Building and fire codes include specific cabling requirements, usually aimed at the combustibility and toxicity of the jacket and insulation covering most cables. Polyvinyl chloride (PVC) covers the cheapest and most common cables (for example, the 120-volt cord in lamps and other household appliances). Unfortunately, when this material burns, it gives off toxic fumes, which makes it unsuitable for cables strung in ceilings or inside walls. The space between a false ceiling and the true ceiling in most office buildings, called the "plenum," is commonly used to aid air circulation for heating and cooling. Any cables in this space must be plenum-rated, which typically means they're coated with Teflon because of its low combustibility and the nontoxic fumes it produces when burned. These cables can be used in the plenum or inside walls without being enclosed in conduit. Although plenum-rated cable is nearly twice as expensive as non-plenum-rated cable, eliminating the need for conduit makes installing plenum-rated network cabling much cheaper. UTP cabling is usually marked as communication cable riser (CMR) or communication cable plenum (CMP). CMR is suitable only for building risers, such as elevator shafts or in cable trays, and can't be used in spaces that carry environmental air. CMP is suitable for use in plenum spaces. Before installing any type of cable, check all local fire and building codes because requirements vary widely.
Processing Components
CPU Output Components-printers, monitors, storage devices, network cards, and speakers RAM CD /DVD's
Wireless Extended LAN Technologies
Certain kinds of wireless networking equipment extend LANs beyond their normal cable-based distance limitations or provide connectivity across areas where cables are not allowed (or able) to traverse. For instance, wireless bridges can connect networks up to 3 miles (4.4 km) apart. These LAN bridges per- mit linking locations by using line-of-sight or broadcast transmissions. They can also make it unnecessary to route dedicated digital communication lines from one site to another through a communications carrier. Normally, upfront expenses for this technology are as much as 10 times higher, but it eliminates recurring monthly service charges from a carrier. This savings can quickly make up for (and exceed) the initial expense. Spread-spectrum radio, infrared, and laser-based equipment are readily available commercially. Longer-range wireless bridges are also available, including spread-spectrum solutions that work with Ethernet or token ring over distances up to 25 miles. As with shorter-range wire- less bridges, the communication cost savings over time can justify the cost of a long-range wireless bridge. When it's connected correctly, this equipment (in long-range and short- range varieties) can transport both voice and data traffic. Table 4-6 summarizes the charac- teristics of wireless extended LAN technologies.
Ipconfig/all
Displays all current TCP/IP network configuration values and refreshes Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS) settings. Used without parameters,
Ipconfig
Displays the IP address, subnet mask, and default gateway for all adapters.
Domain Name System
Domain Name System (DNS) is a name-to-address resolution protocol that keeps a list of computer names and their IP addresses. Through a correctly configured workstation, a user can use a computer's name—for instance, Server1 or www. course.com—rather than a numerical address, such as 207.46.134.189, to communicate with the computer. For example, when you enter "www.course.com" in your Web brow- ser's address box, the Web browser contacts the DNS client service on your computer. The DNS client contacts the DNS server specified in your OS's IP configuration and requests that the name "www.course.com" be resolved to an IP address. The DNS server responds with the IP address assigned to the computer named www at the course.com domain. From there, using the IP address returned, your Web browser application can contact the Web server to request a Web page. DNS uses the UDP Transport-layer protocol because DNS messages usually consist of a single packet of data, so there's no need for the reliability measures TCP offers. The DNS system used throughout the Internet is organized as a treelike hierarchy (see Figure 5-9). The tree consists of these domain levels: root, top, second, subdomain, and host. All levels below the root level have branches, each of which has a name. When you put all the names of a branch together, separated by a period, you have the fully qualified domain name (FQDN) of the network resource, such as www.course.com.
Link Status Light
Examine the hub's indicator lights. A link status light should be glowing for each port a computer is connected to. Next, examine the indicator lights on the NIC, which should also be glowing to indicate a good connection. See whether the hub's indicator lights vary for different connection speeds. Write the link status light's color and the connec- tion speed, if available, in the following chart:
Fiber-Optic Cable
Fiber-optic cable trades electrical pulses for pulses of light to represent bits. Because no electrical signals ever pass through the cable, fiber-optic media is as immune to electrical interference as any medium can get. Therefore, light pulses are unaffected by EMI and RFI. This characteristic also makes fiber-optic cables highly secure. They emit no external signals that might be detected, unlike electrical or broadcast media, thereby eliminating the possibility of electronic eavesdropping. In particular, fiber-optic cable is a good medium for high-bandwidth, high-speed, long- distance data transmission because of its lower attenuation characteristics and vastly higher bandwidth potential. Today, commercial implementations at 10, 40, and 100 Gbps are in use.
Full Duplex
Full-duplex mode, by definition, means a NIC can transmit and receive simultaneously. Therefore, when an Ethernet NIC is operating in full-duplex mode connected to a switch, CSMA/CD isn't used because a collision can't occur in full-duplex mode. Because full- duplex mode eliminates the delays caused by CSMA/CD and allows double the network bandwidth, most Ethernet LANs now operate in this mode using switches.
Half-Duplex
Half-duplex communication means a station can transmit and receive data but not at the same time, much like a two-way radio. When Ethernet is implemented as a logical bus topology (using hubs), NICs can operate only in half-duplex mode and must use the CSMA/CD access method.
Hexadecimal Notation
Hexadecimal Notation Hexadecimal notation, which you have seen in MAC addresses and now IPv6 addresses, is a numbering system like decimal and binary. Hexadecimal, or just hex, is based on powers of 16 and uses 16 symbols to represent all possible numbers. Rather than invent new symbols, however, the numbers 0 to 9 are used for the first 10 symbols and the letters A to F for the remaining 6 symbols, which have the values 10 to 15 in decimal. A hex number, therefore, is expressed by using the symbols 0 to F, and each place value is based on a power of 16. For example, the hex number 4C can be converted to decimal by 4×161 +C×160.ThesymbolCrepresentsdecimal12,so4Cindecimalis64+12=76. Hexadecimal notation is often used to represent numbers in the computer world because it can be converted easily to binary, as it's based on powers of 2. For example, 24 = 16, so every hex digit can be expressed as exactly 4 bits. Converting from hex to binary is just a matter of converting each digit to its 4-bit binary equivalent. For instance, AC4F in hexadec- imal is expressed as 1010 1100 0100 1111 in binary. Networking doesn't require a lot of hex-to-binary conversion until you start working with IPv6 addresses. If you're an aspiring programmer, however, understanding the hex number- ing system is sure to be beneficial.
Interference Susceptibility /Eavesdropping Susceptibility
How well a media type resists signal interference from outside sources depends on the medium's construction and type of signals it's designed to carry. Interference to electrical signals on copper media comes in the form of electromagnetic interference (EMI) and radio frequency interference (RFI). Motors, transformers, fluorescent lights, and other sources of intense electrical activity can emit both EMI and RFI, but RFI problems are also associated with the proximity of strong broadcast sources in an environment (such as a nearby radio or TV station). RFI can also affect wireless networks if the frequencies are in the same range in which the wireless net- work is operating. Another type of interference found in copper wires is a form of EMI called crosstalk, which is interference one wire generates on another wire when both wires are in a bundle (as all cabling in LANs is). When electrical signals travel across the medium, they create their own electromagnetic field. Although this field is weak, it can leak onto other wires, especially when their insulation is in contact with the other wire. Although it's not as common now, you might have experienced crosstalk while talking on a landline phone and hearing another conversation faintly. With phone wires, crosstalk is merely an annoyance because people can filter out this noise easily, but in networking, excessive crosstalk can render the network connection unusable. Because electrical signals traveling down a copper wire create an electromagnetic field that can be detected outside the wires, copper wire is susceptible to electronic eavesdropping. It might sound like the stuff of spy movies, but with the right type of equipment, an eaves- dropper simply needs to get close to a copper cable to extract data from it. In the absence of sensitive electronic equipment, if eavesdroppers have physical access to the connecting equipment and the copper wire is slightly exposed, they would have no problem installing a listening device directly on these wires. Fiber-optic media carries light signals and is impervious to interference. In addition, because no magnetic field is present, eavesdropping is a difficult proposition with fiber-optic cable. To eavesdrop, someone needs access to the glass strands carrying the optical signals to
IP Address Classes
IP addresses are categorized in ranges referred to as Classes A, B, C, D, or E. Only IP addresses in the A, B, and C classes are available for host assignment. Although the IP address class system has been somewhat superseded by a more flexible way to manage IP addresses, called Classless Interdomain Routing (CIDR, discussed later in this chapter in "Classless Interdomain Routing"), the class system is a basis for determining which part of an IP address is the network ID and which part is the host ID. The first octet of an address denotes its class. Note the following facts about IP address classes: ● The value of the first octet for Class A addresses is between 1 and 127. Class A addresses are intended for use by large corporations and governments. An IP address registry assigns the first octet, leaving the last three octets for network administrators to assign to hosts. This allows 24 bits of address space or 16,777,214 hosts per network address. In a Class A IP address such as 21.155.49.211, for example, the network ID is 21.0.0. So the first address in the 21.0.0.0 network is 21.0.0.1, and the last address is 21.255.255.254. ● Class B addresses begin with network IDs between 128 and 191 and are intended for use in medium to large networks. An IP address registry assigns the first two octets, leaving the third and fourth octets available for administrators to assign as host addresses. In the Class B address 172.17.11.4, for example, the network ID is 172.17.0. Having two octets in the host ID allows 65,534 hosts per network address. ● Class C addresses are intended for small networks. An IP address registry assigns the first three octets, ranging from 192 to 223. In the Class C address 211.255.49.254, for example, the network ID is 211.255.49. These networks are limited to 254 hosts per network. ● Class D addresses are reserved for multicasting, in which a packet is addressed so that more than one destination can receive it. Applications using this feature include video- conferencing and streaming media. In a Class D address, the first octet is in the range 224 to 239. Class D addresses can't be used to assign IP addresses to host computers. ● Class E addresses have a value from 240 to 255 in the first octet. This range of addresses is reserved for experimental use and can't be used for address assignment.
Default Gateway
In a computer's IP address settings must be set to the address of a router to which the computer can send all packets destined for other networks. If the default gateway doesn't have a valid address of a router, the computer can communicate only with computers on the same LAN.
Subnet Masks
In the early days of IP, a host used the address class to determine which part of the address was its network ID. However, this method doesn't offer the flexibility that CIDR addressing requires. When an IP address is assigned to a computer or other IP device, it's always accom- panied by a subnet mask. IP uses an address's subnet mask to determine which part of the address denotes the network portion and which part denotes the host. It's a 32-bit number in dotted decimal format consisting of a string of eight or more binary 1s followed by a string of 0s. A binary 1 in the subnet mask signifies that the corresponding bit in the IP address belongs to the network address, and a binary 0 signifies that the corresponding bit in the IP address belongs to the host ID.
Locally Administered Address Property
Locally Administered Address property. (It might also be referred to as network address, physical address, or MAC address.) In most cases, this property's value is set to Not Present. You can use this property to override the NIC's burned-in MAC address by entering a new address in the Value text box. Normally, however, you shouldn't override the burned-in MAC address because if you duplicate an exist- ing address accidentally, it can cause a loss of communication. Click Cancel to close Network Connection Properties.
Network Bandwidth
Network bandwidth is the amount of data that can be transferred on a network during a specific interval. It's usually measured in bits per second, and networks operate at speeds from 10 million bits per second (10 Mbps) up to 10 gigabits per second (Gbps).
Medium Dependent Interface
Network devices connecting with RJ-45 plugs over twisted-pair cabling are classified as medium dependent interface (MDI) devices or MDI crossed (MDI-X) devices.
Ethernet Error Handling
One reason for Ethernet's low cost and scalability is its simplicity. It's considered a best-effort delivery system, meaning that when a frame is sent, there's no acknowledgement or verification that the frame arrived at its intended destination. Ethernet relies on network protocols, such as TCP/IP, to ensure reliable delivery of data. It's similar to the package delivery guy at a corporation. His job is to take what he's given to its intended destination; it's the package receiver's job to verify its contents and let the sender know it was received. Ethernet can also detect whether a frame has been damaged in transit. The error-checking code in an Ethernet frame's trailer is called a Cyclic Redundancy Check (CRC), which is the result of a mathematical algorithm computed on the frame data. The CRC is calculated and placed in the frame trailer before the frame is transmitted. When the frame is received, the calculation is repeated. If the results of this calculation don't match the CRC in the frame, it indicates that the data was altered in some way, usually from electrical interference. If a frame is detected as damaged, because Ethernet is a best-effort delivery system, it simply discards the frame but doesn't inform the sending station that an error occurred. Again, it's the network protocol's job to ensure that all expected data was actually received. The net- work protocol or, in some cases, the application sending the data is responsible for resending damaged or missing data, not Ethernet.
Output
The CPU sends instructions to the graphics cards to display the letter A, which is then sent to the computer monitor.
Physical Topologies
The arrangement of cabling and how cables connect one device to another in a network are considered the network's
Processing
The computer's CPU determines what letter was typed by looking up the keyboard code in a table.
Uplink
The connection of multiple hubs
Routes Packets Through an Internetwork
The next task of the Internetwork layer is determining the best way to get a packet from network to network until it reaches its destination. If there were only one way for a packet to get from here to there, this aspect of the Internetwork layer's job would be pretty ho-hum. However, much like the nation's road system, most large networks, such as the Internet, have multiple paths for getting from location A to location B. Which path to take isn't always a clear-cut decision. Some paths are heavily traveled, and some are lightly traveled; some paths have construction or accidents, and others are clear sailing. As mentioned, routers work at the Internetwork layer, and their job is to select the best path to the destination. If a path becomes unavailable or congested, they select an alternative, if available. Routers use the network ID portion of IP addresses along with their routing tables to determine on which network a destination device can be found and the best way to get packets to their destination. Chapter 7 discusses routers in more detail.
UTTP
Unshielded Twisted Pair Unshielded twisted pair (UTP) is the most common media type in LANs. It consists of four pairs of copper wire, with each pair tightly twisted together and contained in a plastic sheath or jacket (Figure 3-10)Unshielded Twisted Pair Unshielded twisted pair (UTP) is the most common media type in LANs. It consists of four pairs of copper wire, with each pair tightly twisted together and contained in a plastic sheath or jacket (Figure 3-10).
Establishing a Connection: The TCP Handshake
stablishing a connection with TCP is similar to making a phone call. You dial the number and wait for your party to answer, usually with a greeting. The caller then states his or her name and says who he or she wants to talk to. If everything is agreeable, a conversation begins. A TCP session begins when a client sends a TCP synchronization (SYN) segment to the destination device, usually a server. A destination port number (typically a well-known port, such as 80) is specified, and a source port number is assigned dynamically. When the server receives the SYN segment, it usually responds by sending one of two segments: an acknowledgement-synchronization (ACK-SYN) segment or a reset connection (RST) segment. If an RST segment is returned, the server refused the request to open a session, possibly because the destination port is unknown. If an ACK-SYN segment is returned, the client com- pletes the three-way handshake by sending an ACK segment back to the server. The client is then ready to begin sending or requesting data. You capture and examine a three-way hand- shake in Challenge Lab 5-1.
Wireless Benefits
● Create temporary connections to existing wired networks. ● Establish backup or contingency connectivity for existing wired networks. ● Extend a network's span beyond the reach of wire-based or fiber-optic cabling, especially in older buildings where rewiring might be too expensive. ● Allow businesses to provide customers with wireless networking easily, thereby offering a service that gets customers in and keeps them there. ● Enable users to roam around a corporate or college campus with their machines.
Infrared LAN Technologies
● Line-of-sight networks require an unobstructed view, or a clear line of sight, between the transmitter and receiver. ● Reflective wireless networks broadcast signals from optical transceivers near devices to a central hub, which then forwards signals to their intended recipients. ● Scatter infrared networks bounce transmissions off walls and ceilings to deliver signals from sender to receiver. TV remotes work in this fashion. This approach limits maximum reception distances to approximately 30 meters (100 feet). Because bounce technologies introduce signal delays, scatter infrared results in lower bandwidth than line of sight. ● Broadband optical telepoint networks provide broadband services. This technology offers high speed and wide bandwidth, can handle high-end multimedia traffic, and matches the capabilities of most wired networks.
