Autonomous System (AS)

The internet is not just one big network. There are millions of autonomous systems that cooperate to route IP packets. Read this article to fully understand this concept.

What is AS (Autonomous System)?

A portion of a large internetwork that is under a given administrative authority. Autonomous systems can be under the authority of a particular corporation or institution, or they can be defined by the uniform use of a particular routing protocol such as Open Shortest Path First (OSPF). Autonomous systems are part of the routing infrastructure of a large internetwork and can be subdivided into routing domains.

The Internet is the prime example of a large internetwork divided into different autonomous systems (such as CERFnet, SprintLink, and AlterNet). These autonomous systems are connected with backbone routers that use the Border Gateway Protocol (BGP) for communication among them. Each autonomous system is represented by a 16-bit integer assigned by Internet Network Information Center (InterNIC), which is used by BGP to avoid routing loops and implement policy-based routing on the Internet backbone. There are three types of autonomous systems:

  • Stub AS:
    This AS is connected to only one other AS. A corporate network that is connected to an AS is considered to have the same AS number as the AS it is connected to.

     

  • Transit AS:
    This AS is connected to more than one other AS and can be used for transit traffic between autonomous systems. They are usually administered by large Internet service providers (ISPs).

     

  • Multihomed AS:
    This AS is connected to more than one other AS but does not let transit traffic from another AS pass through itself. An example might be a corporate network with several Internet connections to different ISPs.

What is an autonomous system?

Geoffrey Challen, Assistant Professor at the University at Buffalo, explains in a very simple manner the concept of Autonomous System in a large network like the internet.


When discussing routing, we sometimes refer to these independent networks as autonomous systems. Each autonomous system is responsible for routing packets internally. To travel across the broader internet, packets must traverse multiple autonomous systems. So it is more accurate to think of internet routing as happening across autonomous systems, rather than between individual computers.