IP Address Classes

IP addresses are grouped into logical divisions called classes. In the IPv4 address space, there are five address classes (A through E), although only three (A, B, C) are used for assigning addresses to clients. Class D is reserved for multicast addressing, and Class E is reserved for future development.

Of the three classes available for address assignments, each uses a fixed-length subnet mask to define the separation between the network and the node address. A Class A address uses only the first octet to represent the network portion, a Class B address uses two octets, and a Class C address uses the first three octets. The upshot of this system is that Class A has a small number of network addresses, but each class A address has a very large number of possible host addresses. Class B has a larger number of networks, but each class B address has a smaller number of hosts. Class C has an even larger number of networks, but each Class C address has an even smaller number of hosts. The exact numbers are provided in Table 2.

Table 2 IPv4 Address Classes and the Number of Available Network/Host Addresses
Address Class	Range	Number of Networks	Number of Hosts per Network	Binary Value of First Octet
A	1126	126	16,777,214	0xxxxxxx
B	128191	16384	65,534	10xxxxxx
C	192223	2,097,152	254	110xxxxx
D	224239	NA	NA	1110xxxx
E	240255	NA	NA	1111xxxx

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