edVirtus Reference

What Is an Autonomous System?

How Does an Autonomous System Organize Routing on the Internet?

An Autonomous System (AS) is a collection of interconnected Internet Protocol (IP) networks and routers that are managed by a single organisation and operate under a common routing policy. Autonomous Systems form the fundamental building blocks of the Internet, allowing millions of individual networks to exchange traffic in an organised and scalable manner.

The basic principle is straightforward. Within an Autonomous System, routers exchange routing information using an Interior Gateway Protocol (IGP) such as Open Shortest Path First (OSPF) or Intermediate System to Intermediate System (IS-IS). When communicating with other Autonomous Systems, specially designated border routers exchange routing information using the Border Gateway Protocol (BGP). Each Autonomous System therefore manages its own internal routing while advertising the networks it can reach to neighbouring Autonomous Systems.

A useful analogy is a country's road authority. Each state or province manages its own internal road network according to its own policies and priorities, but major highways connect neighbouring states to create a nationwide transportation system. Similarly, each Autonomous System manages its own internal network while cooperating with others to form the global Internet.

Every Autonomous System is identified by a unique Autonomous System Number (ASN) assigned by a regional Internet registry. The ASN is used by BGP to identify the source and destination of routing information exchanged between Autonomous Systems. Modern ASNs are 32 bits long, allowing billions of unique identifiers, although many smaller private networks use private ASNs that are not advertised across the public Internet.

Autonomous Systems are generally classified according to how they connect to other networks. A stub AS connects to only one external Autonomous System and carries no transit traffic. A multihomed AS connects to two or more external networks for improved reliability but does not normally carry third-party traffic. A transit AS, typically operated by a large Internet service provider, carries traffic between other Autonomous Systems and forms part of the Internet backbone.

It is important to distinguish an Autonomous System from a routing protocol. An AS is an administrative and routing domain, whereas protocols such as OSPF and IS-IS manage routing within an AS, and BGP manages routing between Autonomous Systems. Likewise, an AS differs from a subnet, which is simply a subdivision of an IP network within a single administrative domain.

Today, more than one hundred thousand Autonomous Systems cooperate to form the global Internet. By dividing the Internet into independently managed routing domains connected through BGP, the Autonomous System architecture provides the scalability, flexibility, and administrative independence needed to support worldwide communication between billions of devices.