Classful network

A classful network is a network addressing architecture used in the Internet from 1981 until the introduction of Classless Inter-Domain Routing in 1993. The method divides the address space for Internet Protocol Version 4 (IPv4) into five address classes by address range. Classes A, B, C are networks of three different network sizes, i.e. number of hosts for unicast addresses. Class D is for multicast. The class E address range is reserved for future or experimental purposes. Under classful networking, the subnet mask was implied by which address range (class) the address occupied and did not need to be specified separately.

Since its discontinuation, remnants of classful network concepts remain in practice only in limited scope in the default configuration parameters of some network software and hardware components, most notably in the default configuration of subnet masks.

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.

Expansion of the network had to ensure compatibility with the existing address space and the (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.

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