What is an ASN, and who owns this IP address?

The short answer: An ASN (Autonomous System Number) identifies a network that runs its own routing on the internet, such as an internet provider, a hosting company, a cloud platform or a content delivery network. Every public IP address sits in a block that one of these networks announces to the rest of the internet. Look up an address's ASN and you find out which network it belongs to, which is usually the quickest answer to "who owns this IP address?"

This guide explains what autonomous systems are, how IP addresses map to them, what the lookup can and can't tell you, and what to do with the answer.

What an autonomous system is

The internet is made of tens of thousands of separate networks that agree to carry traffic for each other. Each network that controls its own routing is an autonomous system, and each gets a number, such as AS15169 (Google) or AS2856 (BT). Networks use a protocol called BGP, defined in RFC 4271, to tell each other which address blocks they can deliver traffic to.

When your laptop sends traffic to a website, it passes from your provider's network through one or more other networks until it reaches the network that announced the website's address block. Each network only needs to know which neighbour is the best next step, a bit like postcodes routing a letter between sorting offices.

Not every organisation has its own ASN. A small business uses its internet provider's network, and a website on shared hosting uses the hosting company's. An organisation usually gets its own ASN only when it connects to more than one provider, or runs a large network of its own.

Who gives out AS numbers

AS numbers are handed out in the same way as IP addresses. IANA allocates blocks of numbers to the five regional internet registries, and they assign them to networks in their region:

• RIPE NCC: Europe, the Middle East and parts of Central Asia, including the UK.
• ARIN: the United States, Canada and parts of the Caribbean.
• APNIC: Asia and the Pacific.
• LACNIC: Latin America and the rest of the Caribbean.
• AFRINIC: Africa.

The registries keep public records of who holds each number and each address block. ARIN's guide to AS numbers explains the usual reasons a network needs one.

16-bit and 32-bit numbers

AS numbers were originally 16 bits, allowing up to 65,535. As they ran short, RFC 6793 extended them to 32 bits, allowing over four billion. Newer networks often have long numbers such as AS200000 or above; they work in exactly the same way.

Reserved numbers

Some numbers never appear on the public internet. RFC 6996 reserves 64512 to 65534 and 4200000000 to 4294967294 for private use inside an organisation's own network, much like private IP ranges. 64496 to 64511 and 65536 to 65551 are for documentation and examples (RFC 5398), which is why this guide uses AS64500.

What an ASN lookup tells you

Enter an IP address or a domain name in our ASN Lookup and you see:

• the AS number and the network's name, such as "Amazon.com, Inc." or "Cloudflare, Inc.";
• the network's domain, which usually tells you who to contact;
• the address block the address sits in;
• the approximate country of the address.

For a domain, we look up its IPv4 and IPv6 addresses and show the network for each, grouped together. A domain can point to more than one network, for example IPv4 at one provider and IPv6 at another, or servers in two clouds.

The data comes from IPinfo's free IPinfo Lite database, which we keep on our own server and update regularly, so the address or domain you enter isn't sent to IPinfo. You can't yet look up an AS number itself to see all the address blocks it holds; that is planned for a later update.

Why a website shows a CDN, not your host

Look up a website behind a content delivery network or proxy, such as Cloudflare, Fastly or Akamai, and you'll see the CDN's network, not your hosting company's. Visitors connect to the CDN's servers, which fetch pages from your real server behind the scenes. That's by design: it hides your server's address and spreads the load.

The same happens with large cloud platforms. A site on Amazon Web Services, Microsoft Azure or Google Cloud shows the cloud provider's network, even though the site belongs to someone else. To find out who runs the site itself, you need other clues, such as the domain's registration or the site's own contact page.

Using the answer

Reporting abuse

If an address is sending spam, attacking your server or hosting a phishing page, the network that owns it is the right place to report it. The regional registries publish an abuse contact for each address block in their RDAP and WHOIS records. Report to that address, with times (including the time zone), the IP address and what happened. Don't report to the CDN or cloud provider's general support unless their abuse page tells you to.

Checking where traffic comes from

Server logs and analytics show IP addresses. An ASN lookup turns them into something readable: "this traffic is from a cloud provider" often means automated bots rather than people, and "this is a mobile network" means visitors on phones. Some firewalls and bot filters can block or allow whole networks by ASN.

Troubleshooting email and connections

If mail from your server is rejected, or a site is unreachable from one provider, knowing the networks involved helps you ask the right people. Check the reverse DNS name of the address too: a mail server should have one that matches its own name. Our guide on reverse DNS for mail servers explains why.

What ASN data can't tell you

• Exact location. The country is approximate, and an ASN says nothing about a street address. Large networks span many countries.
• The person behind an address. Only the network operator knows which customer used an address at a given time, and only shares that with the police or a court.
• Whether an address is safe. Good and bad traffic come from the same big networks. An ASN is a clue, not a verdict.
• Your own server behind a CDN. The lookup shows what the public sees.

Common questions

What does AS15169 mean?

It's the number of one of Google's networks. The "AS" just stands for autonomous system; the number is what routers use.

Why does one company have several ASNs?

Large companies often run separate networks for different services or regions, or acquired other companies' networks. Amazon and Microsoft, for example, have many.

Can an IP address move to another network?

Yes. Address blocks are transferred between organisations, and a network can start announcing a block it has leased. Databases catch up within days or weeks, so a recent move may show the old owner for a while.

Do I need my own ASN?

Almost certainly not, unless you connect to two or more internet providers and want to keep the same addresses if one fails. Your provider or host handles routing for you.

Checklist

• Look up the IP address or domain to find its network, block and approximate country.
• Remember that a CDN or cloud platform shows its own network, not the site owner's.
• For abuse, use the abuse contact the registry publishes for the address block.
• Treat the country as approximate and the network as a clue, not proof.
• Check reverse DNS as well when investigating a mail server.