Autonomous System Number (ASN)
Also known as: AS number, autonomous system
A globally unique number identifying a network (autonomous system) that runs its own routing policy on the internet.
An Autonomous System (AS) is a collection of IP prefixes under a single administrative routing policy - typically an ISP, hosting provider, enterprise, or content network. Each AS is assigned a unique Autonomous System Number (ASN) by a Regional Internet Registry.
ASNs are how BGP, the internet's routing protocol, identifies who originates and forwards a route. Original ASNs were 16-bit (0-65535); the space was extended to 32-bit (up to 4294967295) once the smaller pool ran low. When you see a route in the global table, its origin ASN tells you which network is announcing that address space.
You can look up any ASN to see the prefixes it announces, its RPKI and ASPA status, its upstream providers, and registry details.