In the Internet, an autonomous system (AS) is a collection of Internet Protocol networks and routers, under the control of one or more entities, that presents a common routing policy to the Internet. See RFC 1930 for additional detail on this updated definition.

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. See RFC 1771, the rather obsolete definition of the Border Gateway Protocol, for this definition. The newer definition of 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 are multiple autonomous systems supported by the ISP, the Internet only sees the routing policy of the ISP. That ISP must have a public, registered ASN.

A unique AS number (or ASN) is allocated to each AS for use in BGP Routing. With BGP, AS numbers are important because the ASN uniquely identifies each network on the internet.

=Assignment=

AS numbers are assigned by the Internet Assigned Numbers Authority, which also allocate IP addresses, to Regional Internet Registry (RIRs) in blocks. The local RIR then assigns an AS number to an entity from the block assigned by the IANA. Entities wishing to receive an ASN must complete the application process of their local RIR and be approved before being assigned an ASN. Current IANA ASN assignments can be found on their website: [http://www.iana.org/assignments/as-numbers]

AS numbers are currently 16-bit integers, which allow for a maximum of 65536 assignments. AS numbers are divided into 2 ranges. The first are public AS numbers, which may be used on the Internet and range from 1 to 64511. The second range, from 64512 to 65536, are known as private numbers, and can only be used internally within an organization.

While there is no immediate danger of exhausting the 16-bit AS number space, several factors, principally the need of enterprises to run BGP to multihome, is reason for concern that more space will be needed in the moderate term. As of mid-2005, the Internet Engineering Task Force has several drafts underway that define mechanisms for the use of an upwardly compatible four-octet, or 32-bit, AS number space. This space will not replace the existing 16-bit space.

=Types=

Autonomous Systems can be grouped into 3 categories, depending on their connections and operation.

A multihomed AS is an AS that maintains connections to more than one internet service provider. This allows the AS to remain connected to the internet in the event of a complete failure of one of their ISPs. In addition, this type of AS would not allow traffic from one ISP to pass through on its way to another ISP.

A stub AS refers to an AS that is only connected to a single ISP. This is generally regarded to be a needless use of an AS number, as the network is completely under a single ISP, and thus does not need to be uniquely identified.

A transit AS is an AS that provides connections through itself to the networks connected to it. That is, network A can use the transit AS to connect to network B. ISPs are always transit ASs, because it is their business to connect disparate networks in exchange for money. The ISP is considered to be selling transit service to the end network, thus the term transit AS .

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