ASN Lookup: Find ASN by IP or Domain - Free

Use this free ASN lookup to find the autonomous system number behind an IPv4 address, a domain, or another ASN. Enter one value and the tool shows which network owns it, the AS name or holder, and, for IPs and domains, the hostname, location and timezone. It suits site owners, sysadmins and researchers who need to know who runs a network.

What Is an ASN (Autonomous System Number)?

An autonomous system number is a unique ID assigned to a network, or a group of networks, that follows a single routing policy. Internet providers, cloud hosts, universities and large companies use ASNs so they can exchange routes with other networks through the Border Gateway Protocol (BGP).

Think of an ASN as the public ID of a network, and an IP address as one street address inside it. The five regional internet registries (ARIN, RIPE NCC, APNIC, LACNIC and AFRINIC) hand out ASNs. They are written with an AS prefix, such as AS15169. Early ASNs were 16-bit numbers, and the range was later extended to 32-bit, which is why some ASNs have up to ten digits.

ASN vs. IP address

One ASN usually covers many blocks of IP addresses, called prefixes. A hosting company may announce thousands of addresses under a single ASN. That is why an IP lookup returns an ASN: it tells you whose network the address sits in.

Not the other ASN

The same letters mean other things elsewhere. In supply chain work, ASN is an advance shipping notice, and in education it is an associate degree in nursing. This page covers the networking meaning only.

How to Use the ASN Lookup Tool

You can run a lookup in four short steps, and each one matches a control on the page.

1. Click the field labeled IP Address, Domain, or ASN.
2. Type an IPv4 address such as 8.8.8.8, a domain such as example.com, or an ASN such as AS15169. If you paste a full web address, the tool uses the domain part.
3. Click Lookup ASN. The button changes to Looking up... while the request runs.
4. Read the result list below the form, then copy the ASN or holder name for your notes, a firewall rule or a support ticket.

You can type an ASN with or without the AS prefix. Both 15169 and AS15169 are accepted. Leaving the field empty returns a message asking you to enter a value.

Worked Example: Looking Up 8.8.8.8

Entering 8.8.8.8 shows the network that owns the address. You would typically see the IP address, a hostname such as dns.google, the ASN AS15169, an AS name tied to Google, and a location in the United States with its timezone.

Here is how to read it. The ASN tells you the address belongs to Google's network, not to a small hosting reseller. The hostname confirms this, because it points back to a Google service. If you found this IP in your server logs, you would know the traffic comes from a large, well-known operator.

Now try the reverse. Entering AS15169 returns the ASN, the holder (the organization that registered it) and an IPv4 prefix line. A Yes there means the network is currently announced in global routing. A No means it is not being announced at the moment, so the number may be unused or retired.

How Do You Read the Results?

The fields you see depend on what you entered. An IP or domain returns the fuller set, while an ASN returns a shorter one.

Only fields that have data appear, so a missing hostname or postal code is normal, not an error.

What a good result looks like

For a site you run, the ASN and AS name should match the host or CDN you pay for. If you use a CDN, you may see the CDN's ASN instead of your origin host, which is expected because visitors reach the CDN first.

What a poor or surprising result looks like

• An unknown or unexpected owner: a server you thought was on your hosting plan shows another company. Check your DNS records, since an old record may point to a previous provider.
• Location far from your audience: the IP is based in another region. Consider a host or CDN closer to your visitors.
• Prefixes announced: No: the ASN is not currently visible in routing. Confirm you typed the right number.

When Should You Use an ASN Lookup?

Use an ASN lookup whenever you need to know who operates the network behind an address. These are the situations where it saves time.

• Identifying a hosting provider. Enter any domain to see which network serves it. For the plain IP and server details, a domain to IP lookup pairs well with the ASN.
• Reviewing server logs. Look up an IP that sends heavy or odd traffic. If it belongs to a cloud or hosting ASN rather than a residential provider, it is more likely an automated client.
• Writing firewall or rate-limit rules. Knowing the ASN helps you decide whether a rule should target one address or a whole network.
• Spotting shared infrastructure. If several of your sites, or sites you link to, sit in one network, a Class C IP checker shows how closely their IPs cluster.
• Migrating hosts. Check the ASN before and after a move to confirm your domain now points to the new provider.
• Researching a network. Enter an ASN from a security report or peering document to see who holds it.

Best Practices and Troubleshooting Tips

Most bad results come from the input, not the network. These checks fix the common ones.

• Type the domain, not a page path. Use example.com rather than example.com/blog/post. The tool trims paths, but a clean domain avoids typos.
• Fix the Could not resolve message. This means the domain did not return a public IP address. Check the spelling, then run a DNS lookup to confirm the domain has address records. For details on the IPv4 record specifically, see the A record lookup.
• Retry if the lookup is unavailable. Occasionally no result can be verified. Wait a moment and click Lookup ASN again.
• Use the IPv4 form you were given. The examples and hints use IPv4 addresses such as 8.8.8.8. If a domain only has an IPv6 address, check its records with an AAAA record lookup first.
• Check more than one IP. A domain can resolve to several addresses. If you have a list, a bulk IP lookup lets you review up to 20 at once.
• Compare with CDN settings. If the ASN belongs to a CDN, look up the origin server's IP separately to find the real host.

Limits to Keep in Mind

The tool answers one question well: which network does this IP, domain or ASN belong to. It has clear limits, and knowing them keeps you from drawing the wrong conclusion.

• It does not search by company name. You need an IP, a domain or an ASN. If you only know a company name, find one of its domains first and look that up.
• Location is approximate. City, region and postal code describe where an IP is believed to be, often the provider's point of presence, not a street address or a person.
• A domain gives you one answer. Domains behind load balancers or CDNs can resolve differently by region or over time.
• ASN input returns a short summary. You get the holder and the announcement status, not a full list of prefixes. To work with a specific block, convert it with a CIDR to IP range converter.

An ASN shows who runs a network. It does not prove who is behind a particular visitor, and it says nothing about content or intent.

Paste an IP, a domain or an AS number into the box above and click Lookup ASN to see who owns the network. For related checks, browse the full Network and IP tools collection.