Autonomous system (Internet)

In the Internet, an autonomous system (AS) is a collection of IP networks and routers under the control of one entity (or sometimes more) 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 original definition (now obsolete) of the Border Gateway Protocol (BGP). 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. The IANA has reserved ASN numbers 64512 through 65534 to be used for private purposes. The ASNs 0 and 65535 are reserved by the IANA and should not be used in a live environment.