An AS number, or autonomous system number, is a unique number assigned to an autonomous system (AS) by the Internet Assigned Numbers Authority (IANA). An autonomous system is a group of networks that are managed and operated by the same entity, such as a large organization or an Internet service provider (ISP).
The AS number is used to identify the autonomous system and to establish routing between the networks within the AS and the networks in other ASes on the Internet. An AS number is a two-byte (16-bit) number, typically written in decimal notation, such as “AS 65001.”
AS numbers are used by Border Gateway Protocol (BGP), which is the routing protocol used to exchange routing information between autonomous systems on the Internet.
Here’s an example of an AS number:
AS64512 (private AS number).
In this example, “AS64512” represents an autonomous system number. The actual AS numbers can range from 1 to 65535, with numbers from 64512 to 65534 reserved for use in private networks or documentation purposes. AS numbers from 1 to 64511 and 65535 are assigned by regional internet registries (RIRs) to organizations that operate autonomous systems and participate in internet routing.
AS numbers are used in BGP to facilitate routing between autonomous systems. They help in identifying and distinguishing different networks and allow routers to make routing decisions based on the policies and preferences of the autonomous systems.