What Is an ASN?

Learn what an Autonomous System Number is, how BGP uses ASNs, and why IP lookup tools show network ownership and routing information.

An Autonomous System Number, or ASN, identifies a network that presents a consistent routing policy to the internet. Large providers, cloud platforms, universities, content networks, and many organisations operate autonomous systems.

IP lookup tools often show an ASN because it helps identify which network announces an address range. It is more useful for routing and ownership context than for identifying an individual user.

Autonomous systems and BGP

Border Gateway Protocol exchanges routes between autonomous systems. Each route can include an AS path showing the networks used to reach a destination.

What an ASN lookup reveals

An ASN result may show the registered network name, organisation, country, and announced IP prefixes. Records can represent an ISP, hosting provider, enterprise, or transit network.

Why one organisation may have several ASNs

Networks may separate regions, products, acquisitions, or routing policies. Conversely, many brands may use infrastructure announced by one upstream ASN.

Limits of ASN data

An ASN identifies routing responsibility for an address block. It does not prove the identity, intent, or exact location of a person using an IP address.

Frequently Asked Questions

What does AS before a number mean?

It indicates an Autonomous System Number, such as AS15169.

Is an ASN the same as an ISP?

Not exactly. ISPs often operate ASNs, but cloud, enterprise, academic, and content networks do too.

Can an IP move to a different ASN?

Yes. Routing, ownership, leasing, or provider changes can alter which ASN announces a prefix.