Classful network

Classful network is a term that is used to describe the network architecture of the Internet until around 1993. It divided the address space for Internet Protocol Version 4 (IPv4) into five address classes. Each class, coded by the first three bits of the address, defined a different size or type (unicast or multicast) of the network.

Today, remnants of classful network concepts remain in practice only in a limited scope in the default configuration parameters of some network software and hardware components (e.g. netmask), but the terms are often still heard in general discussions about network structure among network administrators.

Map of the prototype Internet in 1982, showing 8-bit-numbered networks (ovals) only, interconnected by routers (rectangles).

Background

Originally, a 32-bit IPv4 address was logically subdivided into the network number field, the most-significant 8 bits of an address, which specified the particular network a host was attached to, and the local address, also called rest field (the rest of the address), which uniquely identifies a host connected to that network. This format was sufficient at a time when only a few large networks existed, such as the ARPANET which was assigned the network number 10, and before the wide proliferation of local area networks (LANs). As a consequence of this architecture, the address space supported only a low number (254) of independent networks, and it became clear very early on that this would not be enough.

Introduction of address classes

Expansion of 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. All existing network numbers at the time were smaller than 64, they only used the 5 least-significant bits of the network number field. Thus it was possible to use the most-significant 3 bits of an address to introduce a set of address classes and preserve the existing network numbers in one of these classes.

The new addressing architecture was introduced by RFC 791 in 1981.^[1] It divided the address space into three address formats, henceforth called address classes.

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 65,536 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.

This addressing scheme is illustrated in the following table:

The number of addresses usable for addressing specific hosts in each network is always 2^N - 2 (where N is the number of rest field bits, and the subtraction of 2 adjusts for the use of the all-bits-zero host portion for network address and the all-bits-one host portion as a broadcast address. Thus, for a Class C address with 8 bits available in the host field, the number of hosts is 254.

Today, IP addresses are associated with a subnet mask. This was not required in a classful network because the mask was implicitly derived from the IP address itself. Any network device would inspect the first few bits of the IP address to determine the class of the address.

The replacement of classes

The first architecture change extended the addressing capability in the Internet, but did not prevent IP address shortage. The principal problem was that many sites needed larger address blocks than a Class C network provided, and therefore they received a Class B block, which was in most cases much larger than required. In the rapid growth of the Internet, the pool of unassigned Class B addresses (2¹⁴, or about 16,000) was rapidly being depleted. Classful networking was replaced by Classless Inter-Domain Routing (CIDR), starting in about 1993, to attempt to solve this problem.

Early allocations of IP addresses by IANA were in some cases not made very efficiently, which contributed to the problem. (However, the commonly held notion that some American organizations unfairly or unnecessarily received Class A networks is wrong; most such allocations date to the period before the introduction of address classes, when the only address blocks available were what later became known as Class A networks.)

Useful tables

Bit-wise representation

In the following table:

• n indicates a binary slot used for network ID.
• H indicates a binary slot used for host ID.
• X indicates a binary slot (without specified purpose)

Class A
  0.  0.  0.  0 = 00000000.00000000.00000000.00000000
127.255.255.255 = 01111111.11111111.11111111.11111111
                  0nnnnnnn.HHHHHHHH.HHHHHHHH.HHHHHHHH

Class B
128.  0.  0.  0 = 10000000.00000000.00000000.00000000
191.255.255.255 = 10111111.11111111.11111111.11111111
                  10nnnnnn.nnnnnnnn.HHHHHHHH.HHHHHHHH

Class C
192.  0.  0.  0 = 11000000.00000000.00000000.00000000
223.255.255.255 = 11011111.11111111.11111111.11111111
                  110nnnnn.nnnnnnnn.nnnnnnnn.HHHHHHHH

Class D
224.  0.  0.  0 = 11100000.00000000.00000000.00000000
239.255.255.255 = 11101111.11111111.11111111.11111111
                  1110XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX

Class E
240.  0.  0.  0 = 11110000.00000000.00000000.00000000
255.255.255.255 = 11111111.11111111.11111111.11111111
                  1111XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX

Special address ranges

Some addresses are reserved for special uses (RFC 3330).^[2]

See also

• Classless Inter-Domain Routing
• List of assigned /8 IP address blocks
• Subnetwork

References

External links

• IANA, Current IPv4 /8 delegations