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, that adhere to a single and clearly defined routing policy.

A unique AS number (or ASN) 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

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