What is an ASN?

The short version

The internet is made of independently operated networks called autonomous systems. Internet service providers, cloud platforms, universities, content networks and large organisations can operate one or more of them.

Each participating autonomous system can be identified by an ASN such as AS15169. Routers exchange reachability information between autonomous systems using BGP, the Border Gateway Protocol.

What does an ASN lookup tell you?

For a public IP address, an ASN lookup can usually tell you which autonomous system currently originates the surrounding address range and the organisation associated with that routing data.

That is useful context when an unfamiliar address belongs to a cloud provider, hosting company, broadband ISP, mobile network or content-delivery network.

An ASN is not the same as a domain owner

If a website resolves to an IP inside a cloud provider's ASN, that does not mean the cloud provider owns the website. It means the IP range is being announced through that network.

The same ASN can contain addresses used by many unrelated customers, and a single organisation can use several ASNs or third-party networks.

Why ASNs are useful during troubleshooting

1. Identify network ownership context. See whether an address sits on residential broadband, cloud hosting or another network type.
2. Compare related addresses. Several IPs in the same ASN can indicate common network infrastructure, although not necessarily the same customer.
3. Understand routing incidents. BGP incidents are often discussed in terms of prefixes and ASNs rather than individual hostnames.
4. Add context to logs. ASN information can make large collections of source IPs easier to understand.

Why ASN data can change

IP ranges can move between networks, organisations can change providers, and routing announcements can change. ASN lookup data should therefore be treated as current network context rather than a permanent identity record.