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), but the terms are often still used in general discussions of network structure among network administrators.
Read more about Classful Network: Background, Introduction of Address Classes, Replacement of Classes
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“How have I been able to live so long outside Nature without identifying myself with it? Everything lives, moves, everything corresponds; the magnetic rays, emanating either from myself or from others, cross the limitless chain of created things unimpeded; it is a transparent network that covers the world, and its slender threads communicate themselves by degrees to the planets and stars. Captive now upon earth, I commune with the chorus of the stars who share in my joys and sorrows.”
—Gérard De Nerval (1808–1855)