Autonomous System Numbers, referred to as AS Numbers (ASN), are 16-bit and 32-bit routing prefix numbers that are used within Border Gateway Protocol (BGP) routing.
The Border Gateway Protocol (BGP) is an application layer protocol of the Internet protocol suite. BGP is defined, within RFC-1771, as an inter-Autonomous System routing protocol. BGP provides a 'classless' system where computer networks can exchange and share data loads. BGP is currently (2014) version 4 (BGP4); BGP4 was codified in RFC-4271
BGP is most commonly used by Internet Service Providers to connect to other Internet Service Providers; so that they can exchange data across each other networks. BGP is essential for multi-honed ISPs: these are ISPs who rent network transit from multiple higher tier ISPs. BGP enable ISPs to exchange data without having to pay for upstream network access, and can save ISPs money. Therefore, BGP is an important protocol for large computer networks, like ISPs, but will not be used by most end users.
Internet Assigned Numbers Authority (IANA) assigns ASNs, and does so by delegating responsibility to five regional Internet registries (RIRs). The RIRs will assign a ASN to computer networks within their regions, which are: