Introduction of address classes

Network & Host

Introduction of address classes

Expansion of the network had to ensure compatibility with the existing address space and the Internet Protocol (IP) packet structure, and avoid the renumbering of the existing networks. The solution was to expand the definition of the network number field to include more bits, allowing more networks to be designated, each potentially having fewer hosts. Since all existing network numbers at the time were smaller than 64, they had only used the 6 least-significant bits of the network number field. Thus it was possible to use the most-significant bits of an address to introduce a set of address classes while preserving the existing network numbers in the first of these classes.

The new addressing architecture was introduced by RFC 791 in 1981 as a part of the specification of the Internet Protocol. It divided the address space into primarily three address formats, henceforth called addressclasses, and left a fourth range reserved to be defined later.

The first class, designated as Class A, contained all addresses in which the most significant bit is zero. The network number for this class is given by the next 7 bits, therefore accommodating 128 networks in total, including the zero network, and including the existing IP networks already allocated. A Class B network was a network in which all addresses had the two most-significant bits set to 1 and 0. For these networks, the network address was given by the next 14 bits of the address, thus leaving 16 bits for numbering host on the network for a total of 65536 addresses per network. Class C was defined with the 3 high-order bits set to 1, 1, and 0, and designating the next 21 bits to number the networks, leaving each network with 256 local addresses.

The leading bit sequence 111 designated an "escape to extended addressing mode", and was later subdivided as Class D (1110) for multicast addressing, while leaving as reserved for future use the 1111 block designated as Class E.

This addressing scheme is illustrated in the following table:-

Class	Leading bits	Size of network number bit field	Size of rest bit field	Number of networks	Addresses per network	Start address	End address
Class A	0	8	24	128 (27)	16,777,216 (224)	0.0.0.0	127.255.255.255
Class B	10	16	16	16,384 (214)	65,536 (216)	128.0.0.0	191.255.255.255
Class C	110	24	8	2,097,152 (221)	256 (28)	192.0.0.0	223.255.255.255
Class D (multicast)	1110	not defined	not defined	not defined	not defined	224.0.0.0	239.255.255.255
Class E (reserved)	1111	not defined	not defined	not defined	not defined	240.0.0.0	255.255.255.255