AS number
also: ASN, Autonomous System Number
An Autonomous System (AS) number is a unique identifier assigned to a network or group of networks that operates under a single administrative authority and uses a consistent routing policy on the internet.
An AS number (ASN) is a globally unique 16-bit or 32-bit numerical identifier that identifies an autonomous system—a collection of IP networks and routers controlled by a single organization or service provider. Each AS announces its routes to the internet using the Border Gateway Protocol (BGP).
AS numbers are assigned by regional internet registries (RIRs) such as ARIN, RIPE, and APNIC. For example, a large ISP might be assigned AS number 1234, while a university network might have AS number 5678. These numbers allow routers across the internet to understand which networks belong together and to make routing decisions based on AS-level policies.
In practice, when you run traceroute or use BGP analysis tools on a Linux system, you may see AS numbers and AS paths that show which autonomous systems your traffic traverses. This information is crucial for network administrators managing multi-homed networks or troubleshooting internet connectivity issues.