What Is An Autonomous System?

Date First Published: 2nd April 2022

Topic: Computer Networking

Subtopic: Network Setups

Article Type: Computer Terms & Definitions

Difficulty Level: 6/10

Learn more about what an autonomous system is in this article.

An autonomous system, often abbreviated to an AS is a large network or group of networks that make up the internet. Each autonomous system has a unique identifier, known as an autonomous system number, which is used by the Border Gateway Protocol. A registry of autonomous system numbers is maintained by IANA. Autonomous systems numbered 1-64511 are available by IANA and autonomous systems numbered 64512 - 65535 are reserved for private use. In addition, it has an external routing policy that is managed by a single user or organisation, such as an ISP, university, large enterprise technology company, or government agency. The internet is broken down into autonomous systems.

All devices that are connected to the internet are connected to an autonomous system. Most autonomous systems connect to multiple autonomous systems. Autonomous systems that connect to only one other AS and shares the exact same routing policy may be known as a subnetwork of the first AS.

Note:

All autonomous systems control a specific set of IP addresses. The range of IP addresses that autonomous systems control is known as IP address space.

Categories

Three categories of autonomous systems exist, which include:

• Multihomed – This autonomous system connects to two or more external autonomous systems. In the event of a failure of one of their connections, the autonomous system would stay connected to the internet. However, this category of autonomous system does not allow traffic from one autonomous system to pass through on its way to another autonomous system.
• Transit – This autonomous system has the capability to route data from one autonomous system to another. It acts as a link between multiple external autonomous systems.
• Single-homed (stub) – This autonomous system connects to only one autonomous system.

Public and Private Autonomous Systems

Autonomous systems can be public or private. In order for data to be exchanged with other autonomous systems on the internet, an autonomous system must be public. Autonomous systems that have public autonomous system numbers routes that come from an autonomous system have routes that are visible to the internet. If an autonomous system communicates with only one provider, it must use the Border Gateway Protocol (BGP) for the purpose of performing these communications. In that case, the autonomous system would be private and the routing policy between the autonomous system and the provider will be hidden from the internet.