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 special 16-bit ASN 23456 ("AS_TRANS") was assigned by IANA as a placeholder for 32-bit ASN values for the case when 32-bit-ASN capable routers ("new BGP speakers") send BGP messages to routers with older BGP software ("old BGP speakers") which do not understand the new 32-bit ASNs.
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. 
AS numbers are assigned in blocks by the Internet Assigned Numbers Authority (IANA) to Regional Internet Registries (RIRs). The appropriate RIR then assigns AS numbers to entities within its designated area 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 the IANA website.
Autonomous Systems can be grouped into three categories, depending on their connectivity and operating policy.
A multihomed Autonomous System is an AS that maintains connections to more than one other AS. This allows the AS to remain connected to the Internet in the event of a complete failure of one of their connections. However, this type of AS would not allow traffic from one AS to pass through on its way to another AS.
A stub Autonomous System refers to an AS that is connected to only one other AS. This may be an apparent waste of an AS number if the network's routing policy is the same as its upstream AS's. However, the stub AS may in fact have peering with other Autonomous Systems that is not reflected in public route-view servers. Specific examples include private interconnections in the financial and transportation sectors.
A transit Autonomous System is an AS that provides connections through itself to other networks. That is, network A can use network B, the transit AS, to connect to network C. ISPs are always transit ASs, because they provide connections from one network to another. The ISP is considered to be 'selling transit service' to the customer network, thus the term transit AS.
See also 
- Routing Assets Database (RADB)
- INOC-DBA a hotline communications system between the network operations centers of major Autonomous Systems
- Administrative distance
- RFC 1930, Section 3
- RFC 1771, original definition (now obsolete) of the Border Gateway Protocol (BGP)
- Autonomous System Numbers
- RFC 5398
- RFC 5396, Textual Representation of Autonomous System (AS) Numbers, G. Huston, G. Michaelson, The Internet Society (December 2008)
- RFC 4893, BGP Support for Four-octet AS Number Space
- "Using AS 23456: How BGP Uses Conversion or Truncation For Compatibility". Retrieved 2012-12-17.
- Tony Bates, Philip Smith, Geoff Huston. "CIDR report". Retrieved 2010-09-17.
- ASN Resource Guide
- AS Number plugin for Mozilla Firefox
- CIDR and ASN assignment report update continuously
- Exploring Autonomous System Numbers
- ASN Searching and Tracing by Fixed Orbit
- ASN Lookup
- ashunt, an AS traceroute utility (part of netsniff-ng)
- Autonomous System Whois Lookup
- what is my asn (shows your current asn)
- Why Autonomous Systems?