What is an ASN? Autonomous Systems, explained

An Autonomous System (AS) is a single connected group of one or more IP prefixes run by one or more network operators that has a single, clearly defined routing policy. Every AS is identified by a number — its ASN (Autonomous System Number).

Why ASNs exist

The internet is not one big network; it is tens of thousands of networks (ISPs, cloud providers, universities, enterprises) that exchange traffic. BGP (Border Gateway Protocol) is the protocol these networks use to advertise “I can reach these IP prefixes, reach me via my ASN.” So an ASN tells you which organisation controls a block of addresses and how that block reaches the public internet.

Who assigns ASNs

Regional Internet Registries (RIRs) — ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC — allocate ASNs in their regions. You can look up any ASN, for example AS15169, to see the operator (here: Google) and the IP ranges it announces.

What an ASN tells you in an IP lookup

When you look up an IP, the ASN answers a more reliable question than the city: who actually controls this address space and how does it reach the internet? City-level geolocation can be approximate; the ASN and routing context explain which provider is behind the address. That is why every IP detail page here leads with the ASN and operator, and links to the ASN page and the IP ranges it owns.

FAQ

Is the ASN the same as the ISP? Often related but not identical. The ASN is the routing identity; the operator may be an ISP, a cloud/hosting provider, or an enterprise. A residential ISP and a datacenter can both have ASNs.

Can one organisation have multiple ASNs? Yes. Large operators may run several ASNs for different regions or business units.