2.1 Classfull Addressing
2.1.1 Internet's Address Architecture
The Internet's address architecture is implemented in IP. IP's original addressing scheme dates back to the early 1980s. At the time, the Internet itself was little more than a semi-public network that interconnected a few dozen universities, research organizations, and government bodies. Each of these entities that connected to the Internet had limited networked computing infrastructures. Typically, these infrastructures consisted of little more than a mainframe computer or a handful of UNIX-based minicomputers. PC applications had yet to mature to a level that was useable in business, and local-area networks (LANs) were in their infancy. Therefore, an internetwork did not require a highly scalable architecture.

The original Internet was given a relatively simple, but compact, two-level hierarchy. The top level was the Internet itself, and the bottom level was the collection of individual networks, with their hosts, that were interconnected via the Internet. This hierarchy is illustrated in the main figure.

The Internet's simple --- but powerful and extensible --- architecture served its user community well.