An Autonomous System Number identifies a network — everything one organization, like an ISP or cloud provider, routes under a single policy — independent of which specific IP addresses it currently holds.

How is an ASN different from an IP address?

An IP address identifies one device or service. An ASN identifies the network operator responsible for a whole block of addresses, and is used to route traffic between networks, not to individual hosts.

The five Regional Internet Registries (ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC) allocate ASNs under IANA's authority, the same bodies that allocate IP address blocks.

What Is an ASN?

Every IP address belongs to a network operated by one organization — an ISP, a cloud provider, a university. An ASN identifies that network on the internet, independent of which specific IP addresses it currently holds.

What an Autonomous System Is

An Autonomous System (AS) is a block of IP address ranges under one organization's routing policy — everything Google operates, for example, or everything a single ISP routes for its customers. The internet is really a network of thousands of these systems, each deciding independently how to route traffic within itself and exchanging routes with its neighbors.

Who Assigns ASNs

ASNs are allocated by the five Regional Internet Registries (ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC) under IANA's authority — the same bodies that allocate IP address blocks. A number identifies the network itself, not a location, so an ASN registered in one region can still announce address space anywhere.

How ASNs Are Used in Routing

Networks announce which IP ranges they own using BGP (Border Gateway Protocol), and every announcement is tagged with the announcing network's ASN. Every router on the internet uses these announcements to build a path — a sequence of ASNs — describing which networks traffic has to cross to reach a destination. An ASN lookup on an IP tells you which of these announcing networks currently holds it.

Why Looking Up an ASN Is Useful

Identify the real hosting provider or ISP behind an IP, even when the reverse DNS name is unhelpful
Group thousands of IPs from an abuse report by network, instead of blocking addresses one at a time
Understand why traffic to a service seems to come from a specific country or region — the ASN's registered range often explains it
Investigate whether two seemingly unrelated domains share infrastructure

A Few Well-Known ASNs

ASN	Organization
`AS15169`	Google
`AS13335`	Cloudflare
`AS16509`	Amazon (AWS)
`AS8075`	Microsoft

Look Up an ASN

PixDNS's ASN tool resolves a domain or IP to its announcing network, including the organization name and the full address range it holds.

Look up an ASN →Next: How IP geolocation works →

Try it on your domain

See the theory in practice — run a ASN / BGP Lookup for your own domain.