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An AS (autonomous system) is a collection of networks, or more precisely, the routers joining those networks, that are under the same administrative authority and that share a common routing strategy. An AS has a single "interior" routing protocol and policy. Internal routing information is shared among routers within the AS, but not with systems outside the AS. However, an AS announces the network addresses of its internal networks to other ASes that it is linked to. On the Internet, an AS is an ISP (Internet service provider), but universities, research institutes, and private organizations also have their own ASes.

A group of autonomous systems that share routing information is called a "confederation of ASes" (an interesting phrase when pronounced incorrectly). Autonomous confederations are assumed to have a higher level of trust protection against routing loops among member systems.

Two Internet RFCs discuss autonomous systems: RFC 1930 (Guidelines for creation, selection, and registration of an Autonomous System, March 1996) and RFC 0975 (Autonomous confederations, February 1986)

According to RFC 1930 , "Without exception, an AS must have only one routing policy. Here routing policy refers to how the rest of the Internet makes routing decisions based on information from your AS." Organizations with ASes are assigned 16-bit numbers by IANA (Internet Assigned Numbers Authority). Note: AS numbers are not related to IP addresses.

Figure A-19 illustrates two autonomous systems. IGPs (Interior Gateway Protocols) are used within the AS and EGPs (Exterior Gateway Protocols) are used between the ASes. IGPs provide basic routing within autonomous systems, while EGP is designed to provide reachability information, both about neighbor gateways and about routes to non-neighbor gateways. The IGPs collect the reachability information and the EGP advertises that information.