An Autonomous System Number (ASN) identifies one administrative network — an ISP, a cloud provider, a large enterprise — that exchanges routing information with the rest of the internet using BGP. Every IP prefix in the global routing table is "announced" by exactly one origin ASN.
ASNs were originally 16-bit (numbers up to 65,535), but the pool ran out in 2007. The IETF added 32-bit ASNs (up to 4,294,967,295), which look the same in dotted form (e.g. "AS396982" for Google Cloud).
When you see an IP labelled "AS15169 Google LLC" or "AS13335 Cloudflare", that's the ASN doing the announcing. IPFerret surfaces the origin ASN on the home page and offers per-ASN landing pages with announced prefix lists.
Try it on IPFerret
See also
- BGPThe routing protocol that holds the internet together — every router announces which IP prefixes its network can reach.
- CIDRA notation for IP address ranges using a prefix length, e.g. 192.168.1.0/24 = 256 addresses.
- RIROne of the five organisations that allocate IP space and ASNs in a defined geographic region.