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.