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. Each class, coded in the first four bits of the address, defines either a different network size, i.e. number of hosts for unicast addresses (classes A, B, C), or a multicast network (class D). The fifth class (E) address range is reserved for future or experimental purposes.

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 (e.g., default subnet mask).

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.