AS — Autonomous System

What is an Autonomous System (AS) in the Internet?

An Autonomous System (AS) is a set of Internet routable IP prefixes belonging to a network or a collection of networks that are all managed and supervised by a single entity or organization. An AS utilizes a common routing policy controlled by the entity. The AS is assigned a globally unique 16 digit identification number一known as the AS number or ASN一by the Internet Assigned Numbers Authority (IANA).

Autonomous systems numbered one to 64511 are available by IANA for global use. The 64512 to 65535 series is reserved for private and reserved purposes. Autonomous Systems were introduced to regulate organizations such as Internet Service Providers (ISP), educational institutions and government agencies.

Border Gateway Protocol (BGP) is the protocol that manages the routed peerings, prefix advertisement and routing of packets between different autonomous systems across the Internet. BGP uses the ASN to uniquely identify each system. In effect, BGP is the routing protocol for AS paths across the Internet.