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 a unique autonomous system number (ASN), for use in Border Gateway Protocol (BGP) routing.
AS 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). The allocated AS numbers are defined as 16-bit and 32-bit integers. The length range of 16-bit is between 0 and 65535, and the length range of 32-bit is between 0 and 4294967294. At present, the 16-bit integers which allowed for a maximum of 65,536 assignments is mostly used.
IP
