Autonomous System Definition

An autonomous system (AS), also sometimes called a routing domain, is a network, or group of networks, that is controlled by a common administrator (or group of administrators) on behalf of a business, university, ISP (Internet service provider) or other organization and that presents a common routing policy to the Internet.

Networks within an autonomous system communicate routing information to each other using an interior gateway protocol (IGP). IGP is a generic term for a routing protocol that is used to exchange routing information among routers in an autonomous system. A routing protocol is a set of rules used by routers to determine the most appropriate paths into which they should forward packets towards their intended destinations. IGPs typically support limited geographical areas. Autonomous systems often use a single routing protocol, although multiple protocols can be used.

Autonomous systems share routing information with other autonomous systems using the border gateway protocol (BGP) in place of the exterior gateway protocol (EGP) that was formerly used. It is expected that in the future the BGP will be replaced with the OSI (open systems interconnection) inter-domain routing protocol (IDRP).

Each autonomous system is assigned a number called an autonomous system number (ASN) that uniquely identifies it on the Internet. ASNs are currently 16-bit integers, which allows for a maximum of 65,536 assignments. They are assigned by the Internet Assigned Numbers Authority (IANA), which also allocates IP addresses.

The core of the Internet is composed of many autonomous systems.

Created November 12, 2005.
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