ASN - Autonomous System Numbers

Autonomous System Number (ASN) is a unique identifier assigned to an Autonomous System (AS) on the internet. An AS is a collection of IP networks and routers controlled by single organizations that presents a common routing policy to the internet.

The main purpose of ASNs is to facilitate IP packet routing. Each autonomous system in the global internet has a unique ASN assigned to help identify the routing data traffic between networks. To exchange the information between autonomous systems, a Border Gateway Protocol (BGP) is commonly used.

ASN is a 32-bit integer, but there are two types of ASNs, 2-byte and 4-byte. 2-byte ASNs are traditional ASNs represented by numbers in the range of 1-65,535. 4-byte ASN’s represent numbers in the range from 65,536 – 4,294,967,295 due to the increasing growth of the internet and the need for more unique identifiers.

ASN’s are crucial to the internets infrastructure facilitating the exchange of routing information between networks and enabling the implementation of routing policies. ISP’s and large organizations have their own assigned ASNs typically, allowing them to control the routing of traffic within their networks and between other networks on the internet.