8.1 Autonomous Systems
8.1.1 Segregation of the internet into autonomous systems
BGP is an Exterior Gateway Protocol, or EGP, which was developed to exchange routing information on very large networks, such as the Internet. Generally, each network that connects to a large internetwork is owned and operated by a different entity -- a company, university, government agency, or some other independent group -- which may have unique routing and security policies. Since each of these groups wish to maintain autonomous control over their individual networks (systems), EGPs serve to divide the internetwork up into a series of Autonomous Systems (ASs). Each independent organization that connects to the internetwork typically represents one AS.

During the early days of the Internet, an exterior gateway protocol called EGP version 3 (not to be confused with Exterior Gateway Protocols in general) was used to interconnect these ASs. The National Science Foundation Network (NSFNET) used EGP to exchange reachability information between the backbone and the regional networks. Although EGP was widely deployed, its topology restrictions and inefficiency in dealing with routing loops and setting routing policies created a need for a new and more robust protocol -- so it was replaced with BGP. Currently, BGP version 4 (BGP4) is the accepted standard for Internet routing.