Autonomous System Number (ASN)

On the internet, an autonomous system (AS) is a small unit that has the right to autonomously decide which routing protocol should be used in the system. This network unit can be a simple network or a networking group controlled by one or more common network administrators, it is a single manageable network unit (such as a university, a business, or a company individual). An autonomous system is sometimes referred to as a routing domain. An autonomous system will be assigned a globally unique number, sometimes we call this number an autonomous system number (ASN).

On the internet, an autonomous system (AS) refers to the ensemble of all IP networks and routers under the jurisdiction of one (sometimes multiple) entities that enforces a common routing policy for the Internet. See updated definition in RFC 1930.

Originally, the definition required an autonomous system to be governed by a single entity, usually an internet service provider or a large organization with independent connections to multiple networks, following a single and well-defined routing policy. The newer definition in RFC 1930 is the most widely used since multiple organizations can use their own private AS numbers to run the BGP protocol with the same ISP that connects them to the Internet. Although an ISP supports these multiple autonomous systems, only the ISP's routing policy is visible to the Internet.

The ISP must have a public and officially registered Autonomous System Number (ASN)

Each autonomous system used in BGP routing is assigned a unique autonomous system number (ASN). For BGP, this autonomous system number is very important because the ASN is the unique identifier that distinguishes each network in the entire interconnected network.