ASEs

An Autonomous System (AS) is a collection of IP networks operated by a single administrative entity that follows a unified routing policy for exchanging traffic with other networks. An AS is identified by an Autonomous System Number (ASN), which is allocated by regional Internet registries (RIRs) such as ARIN, RIPE NCC, APNIC, AFRINIC, and LACNIC. AS numbers exist in both 16-bit and 32-bit forms, with 32-bit ASNs widely in use to provide a larger pool of identifiers.

Inter-domain routing on the Internet is governed by the Border Gateway Protocol (BGP), which networks in different ASes use to share reachability information. An AS may announce which IP prefixes it can route and, in turn, learn routes from neighboring ASes. How traffic enters and leaves an AS is determined by routing policies and agreements with other networks.

ASes connect to one another through peering or transit arrangements. Peering involves mutual exchange of traffic without payment, typically at Internet Exchange Points (IXPs) or direct connections. Transit involves paying a provider to carry traffic to and from networks beyond the immediate peers. Large networks may be multihomed, connecting to multiple providers to improve redundancy and performance.

The AS path is a key BGP attribute that lists the sequence of ASes a route traverses, used for loop prevention and policy decisions. ASes vary in size and function, ranging from access networks operated by ISPs to large content delivery networks and enterprise networks. Private use of ASNs is possible for internal routing, while public ASNs are used for Internet-wide routing.