An autonomous system (AS) is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators on behalf of a single administrative entity or domain, that presents a common and clearly defined routing policy to the Internet. Each AS is assigned an autonomous system number (ASN), for use in Border Gateway Protocol (BGP) routing. Autonomous System Numbers are assigned to Local Internet Registries (LIRs) and end user organizations by their respective Regional Internet Registries (RIRs), which in turn receive blocks of ASNs for reassignment from the Internet Assigned Numbers Authority (IANA). The IANA also maintains a registry of ASNs which are reserved for private use (and should therefore not be announced to the global Internet).

Originally, the definition required control by a single entity, typically an Internet service provider (ISP) or a very large organization with independent connections to multiple networks, that adhered to a single and clearly defined routing policy. In March 1996, the newer definition 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 ASN.

Until 2007, AS numbers were defined as 16-bit integers, which allowed for a maximum of 65,536 assignments. Since then, the IANA has begun to also assign 32-bit AS numbers to regional Internet registry (RIRs). These numbers are written preferably as simple integers, in a notation referred to as "asplain", ranging from 0 to 4,294,967,295 (hexadecimal 0xFFFF FFFF). Or, alternatively, in the form called "asdot+" which looks like x.y, where x and y are 16-bit numbers. Numbers of the form 0.y are exactly the old 16-bit AS numbers. 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 first and last ASNs of the original 16-bit integers (0 and 65,535) and the last ASN of the 32-bit numbers (4,294,967,295) are reserved and should not be used by operators; AS0 is used by all five RIRs to invalidate unallocated space. ASNs 64,496 to 64,511 of the original 16-bit range and 65,536 to 65,551 of the 32-bit range are reserved for use in documentation. ASNs 64,512 to 65,534 of the original 16-bit AS range, and 4,200,000,000 to 4,294,967,294 of the 32-bit range are reserved for Private Use.

The number of unique autonomous networks in the routing system of the Internet exceeded 5,000 in 1999, 30,000 in late 2008, 35,000 in mid-2010, 42,000 in late 2012, 54,000 in mid-2016 and 60,000 in early 2018. The number of allocated ASNs exceeded 100,000 as of March 2021.