ASN Lookup

About ASN lookup

ASN Lookup identifies the autonomous system that announces an IP address on the internet, along with the network operator, the announced prefix, the registry country and the allocation date. You can enter an address or a domain, and it is the quickest way to learn which organization actually routes a block of addresses.

How to use it

1. Type an IPv4 address, an IPv6 address or a domain into the IP address or domain box.
2. Press Look up ASN. If you entered a domain, our server resolves it to an address first.
3. Read Autonomous system for the ASN, Network operator and Announced prefix.
4. Check Registration for Country, Registry and Allocated, then the Query card to see the address used and, for a domain, the name it came from.

A well-known resolver address

1.1.1.1

ASN AS13335, network operator CLOUDFLARENET, US, announced prefix 1.1.1.0/24, country US, registry ARIN, allocated 2010-07-14. The query name used is 1.1.1.1.origin.asn.cymru.com.

When to use it

• Working out which provider owns an address that keeps appearing in your firewall logs.
• Finding the ASN of a network before you write a routing filter or a block rule.
• Confirming the announced prefix that contains a given server address.
• Checking whether two addresses belong to the same operator.

Questions about ASN lookup

What is an ASN?

An autonomous system number identifies a network that manages its own routing on the internet, usually an ISP, cloud provider or large company. Prefixes are announced under an ASN, and that is how traffic finds its way.

Where does the data come from?

From an IP-to-ASN mapping service published through DNS TXT records. Your input goes to our server, which resolves any domain to an address and then asks that service for the announcement.

Why do I sometimes see several ASNs?

Some prefixes are announced by more than one autonomous system. All are listed, but the operator name comes from the first only.

Why does it say no announcement was found?

The address is unrouted or reserved, for example a private range, and there is nothing to map. Public addresses that are announced will normally return a result.

What happens with a domain that has both A and AAAA records?

The IPv4 address is used first. It falls back to an IPv6 address only when no A record exists. Requests are capped at 20 per minute per visitor.

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