ASN

An ASN (Autonomous System Number) is a unique identifier assigned to a network — an ISP, cloud provider, university, or large company — that controls its own block of IP addresses and routing policy on the internet. ASNs are what BGP, the protocol that routes traffic between networks, uses to identify who is behind each BGP announcement, telling one operator’s address space apart from another’s. An operator usually announces its whole allocated range as a single aggregated route rather than many small ones, a practice called supernetting. An RPKI ROA ties an ASN to the specific prefixes it’s authorized to originate, giving other networks a signed record to check an announcement against instead of trusting the claimed ASN at face value. A Regional Internet Registry is what actually assigns the ASN in the first place, from the block IANA delegated to it, following the guidelines RFC 1930 sets out for when a network needs its own AS rather than relying on a provider’s — see ARIN’s own guide to requesting an ASN for what that process looks like at one specific registry. The RIPE NCC’s own explainer states the bar at another registry: current RIPE policy requires a network to be multi-homed and to have a unique routing policy before an ASN is assigned. An ASN also forms the first half of a BGP community’s conventional AS:VALUE format, the tag an operator attaches to an announcement to signal routing policy to whoever receives it. Looking up the ASN behind an IP address tells you who actually operates it: a residential ISP, a hosting provider, a mobile carrier, or a cloud platform, which is often more useful than the registered owner name alone when judging whether traffic from an address looks like a home connection, a server, or something else entirely.

AdminKit groups aSN under its IP & network cluster — browse the AdminKit glossary index for other related terms.