Autonomous System (Internet)

Autonomous System (Internet)

Within the Internet, an Autonomous System (AS) is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet.

Originally the definition required control by a single entity, typically an Internet service provider or a very large organization with independent connections to multiple networks, that adhere to a single and clearly defined routing policy, as originally defined in RFC 1771. The newer definition in RFC 1930 came into use because multiple organizations can run BGP using private AS numbers to an ISP that connects all those organizations to the Internet. Even though there may be multiple Autonomous Systems supported by the ISP, the Internet only sees the routing policy of the ISP. That ISP must have an officially registered Autonomous System Number (ASN).

A unique ASN is allocated to each AS for use in BGP routing. AS numbers are important because the ASN uniquely identifies each network on the Internet.

Until 2007, AS numbers were defined as 16-bit integers, which allowed for a maximum of 65536 assignments. The Internet Assigned Numbers Authority (IANA) has designated AS numbers 64512 through 65534 to be used for private purposes. ASNs 64496-64511 are reserved for use in documentation and sample code. The ASNs 0 and 65535 are reserved; ASN 0 may be used to identify non-routed networks. All other ASNs are subject to assignment by IANA. RFC 4893 introduced 32-bit AS numbers, which IANA has begun to allocate. These numbers are written either as simple integers, or in the form x.y, where x and y are 16-bit numbers. Numbers of the form 0.y are exactly the old 16-bit AS numbers, 1.y numbers and 65535.65535 are reserved, and the remainder of the space is available for allocation. The accepted textual representation of Autonomous System Numbers is defined in RFC 5396.

The number of unique autonomous networks in the routing system of the Internet exceeded 5000 in 1999, 30000 in late 2008, 35000 in mid 2010, and 42000 in late 2012.