What Is an ASN (Autonomous System Number)?
An Autonomous System (AS) is a network — or group of networks — under a single administrative and routing policy, such as an ISP, cloud provider or large enterprise. Each AS is identified by a globally unique Autonomous System Number (ASN), written like AS15169.
Why ASNs exist
The Internet is a network of networks. To exchange routes, each network needs a stable identity that does not change when its IP allocations do. ASNs provide that identity in BGP, the protocol networks use to tell each other "to reach these prefixes, send traffic to me."
How they are assigned
| Range | Notes |
|---|---|
| 16-bit (0–65535) | Original space, now exhausted |
| 32-bit (up to ~4.2 billion) | Current space (RFC 6793) |
| 64512–65534, 4200000000+ | Private ASNs (internal use) |
RIRs (ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC) assign public ASNs to networks that need to do their own routing — see IANA and the RIRs.
Look one up
Enter any IP into our ASN lookup to see its origin AS, AS name, BGP prefix, country and RIR. To check the registrant of the address itself, use the IP lookup. To understand how routes flow, read BGP routing basics.
Note: A small business with one ISP does not need its own ASN — it uses the ISP's. You request an ASN when you multi-home (connect to several ISPs) and run BGP yourself.