ASNs

ASNs, or Autonomous System Numbers, identify networks that participate in Internet routing using the Border Gateway Protocol (BGP). An ASN is a globally unique identifier for an autonomous system, a collection of IP networks under a common administrative domain with a single routing policy.

ASNs are allocated and managed by regional Internet registries (RIRs) such as ARIN, RIPE NCC, APNIC, LACNIC, and AFRINIC. Organizations obtain an ASN when they require a distinct routing policy or multi-homing. The RIRs maintain who owns which ASN and publish authoritative records.

In practice, ASNs are used in BGP route advertisements. Each BGP update carries the AS path, listing the sequence of ASNs that a prefix traversed. This path helps routers determine routing loops and apply policies (for example, which routes to accept from certain neighbors or which provider to favor for outbound traffic). Large providers and enterprises often maintain multiple connections and advertise different prefixes to control traffic flow.

There are 16-bit and 32-bit ASNs. Since the introduction of 32-bit ASNs, the available space for unique numbers has increased substantially, helping accommodate the growth of multihomed and large networks. Many modern networking devices support 32-bit ASNs, improving scalability of Internet routing.

Private-use ASNs exist for internal network use and are not intended to be publicly advertised in the global Internet. Organizations may use them within internal routing domains, but such ASNs should not appear in global BGP tables.