In the Internet, an autonomous system (AS) is a collection of IP networks under control of a single entity, typically an Internet service provider or a very large organisation with redundant connections to the rest of the Internet.

A unique AS number is allocated to each AS for use in BGP routing. The numbers are assigned by the same authorities that allocate IP addresses. AS numbers are currently 16-bit integers. There are public numbers, which may be used on the Internet and range from 1 to 64511, and private numbers from 64512 to 65535, which can be used within an organisation.

Types of AS:

  • Multihomed AS
  • Stub AS
  • Transit AS