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| 2.1 |  | IPv4 Addressing |
| 2.1.2 |  | Class A and B IP addresses |

In a class system, IP addresses can be grouped into one of five different classes:

- A
- B
- C
- D
- E

Each of the four octets of an IP address represents either the network portion or the host portion of the address depending on the address class.

Only the first three classes, A, B, and C, are used for addressing actual hosts on IP networks. Class D addresses are used for multicasting. Class E addresses are reserved for experimentation and are not shown in the figure. The following sections explore each of the five classes of addresses.

Class A Addresses
If the first bit of the first octet of an IP address is a binary 0, then the address is a Class A address. With that first bit being a 0, the lowest number that can be represented is 00000000, decimal 0. The highest number that can be represented is 01111111, decimal 127. Any address that starts with a value between 0 and 127 in the first octet is a Class A address. These two numbers, 0 and 127, are reserved and cannot be used as a network address.

Class A addresses were intended to accommodate very large networks, so only the first octet is used to represent the network number. This leaves three octets, or 24 bits, to represent the host portion of the address. With 24 bits total, 224 combinations are possible, yielding 16,777,216 possible addresses. Two of those possibilities, the lowest and highest values, are reserved for special purposes. The low value is 24 zeros and the high value is 24 ones. Therefore, each Class A address can support up to 16,777,214 unique host addresses.

Why are two host addresses reserved for special purposes? Every network requires a network number. A network number is an ID number that is used to refer to the entire range of hosts when building routing tables. The address that contains all 0s in the host portion is used as the network number and cannot be used to address an individual node. 46.0.0.0 is a Class A network number. Similarly, every network requires a broadcast address that can be used to address a message to every host on a network. It is created when there are all 1s in the host portion of the address.

With almost 17 million host addresses available, a Class A network actually provides too many possibilities for one company or campus. Although it is easy to imagine an enormous global network with that many nodes, the hosts in such a network could not function as members of the same logical group. Administrators require much smaller logical groupings to control broadcasts, apply policies, and troubleshoot problems. Fortunately, the subnet mask allows subnetting, which breaks a large block of addresses into smaller groups called subnetworks. All Class A networks are subnetted. If they were not, Class A networks would represent huge waste and inefficiency.

How many Class A addresses are there? Since only the first octet is used as a network number and it contains a value between 0 and 127, 126 Class A networks exist. Each of the 126 Class A addresses have almost 17 million possible host addresses that make up about half of the entire IPv4 address space. Under this system, a mere handful of organizations control half of the available Internet addresses.

Class B Addresses
Class B addresses start with a binary 10 in the first 2 bits of the first octet. Therefore, the lowest number that can be represented with a Class B address is 10000000, decimal 128. The highest number that can be represented is 10111111, decimal 191. Any address that starts with a value in the range of 128 to 191 in the first octet is a Class B address.

Class B addresses were intended to accommodate medium size networks. Therefore, the first two octets are used to represent the network number, which leaves two octets or 16 bits to represent the host portion of the address. With 16 bits total, 216 combinations are possible, yielding 65,536 Class B addresses. Recall that two of those numbers, the lowest and highest values, are reserved for special purposes. Therefore, each Class B address can support up to 65,534 hosts. Though significantly smaller than the networks created by Class A addresses, a logical group of more than 65,000 hosts is still unmanageable and impractical. Therefore, like Class A networks, Class B addresses are subnetted to improve efficiency.

There are 16,384 Class B networks. The first octet of a Class B address offers 64 possibilities, 128 to 191.The second octet has 256 possibilities, 0 to 255. That yields 16,384 addresses, or 25 percent of the total IP space. Nevertheless, given the popularity and importance of the Internet, these addresses have run out quickly. This essentially leaves only Class C addresses available for new growth.

| Web Links P Addressing and Subnetting for New Users http://www.cisco.com/warp/public/701/3.pdf