🏢 ASN Lookup

Look up Autonomous System Number (ASN) information for IP addresses or find details about specific ASNs.

Enter IP Address or ASN:

💡 Tip: Enter an IP address or ASN and click "Lookup ASN" to retrieve information.

Enter an IP address or ASN and click "Lookup ASN" to view results.

What the lookup asks and what comes back

Anything beginning with the letters AS is treated as an AS number and the prefix is stripped before the query; everything else is sent as an IP address. The question is then put to Team Cymru's whois service, which answers from the global BGP table rather than from a registry:

AS15169   -> queried by number
8.8.8.8   -> queried by address

returned: asn | organization | country | announced prefix

Look up 8.8.8.8 and it resolves to AS15169, Google, announced prefix 8.8.8.0/24 - a 256-address block. The prefix is the field that matters: it is the CIDR entry other routers actually hold, so it marks the real boundary of the network that address sits in.

Prefixes, aggregation and what gets accepted

Routers use longest-prefix match. If both 8.8.8.0/24 and 8.8.0.0/16 exist in the table, traffic to 8.8.8.8 follows the /24, because the more specific route always wins regardless of the order they arrived.
AS numbers are 32-bit since RFC 6793. The original range was 0 to 65,535; 64,512 to 65,534 and 4,200,000,000 to 4,294,967,294 are reserved for private use and never appear on the public internet.

Most transit providers filter anything longer than a /24 in IPv4 or a /48 in IPv6, so splitting a /24 into two /25s gives you two blocks that are valid subnets but will not be carried globally. Only one result is shown even when an address is announced from more than one AS.

Frequently asked questions

How do I find the ASN for an IP address?

Paste the address in. The lookup reports the autonomous system currently originating a route that covers it, along with the operator name, country and the prefix being announced.

Why is the returned prefix larger than my own subnet?

Because BGP carries the aggregate, not your internal split. A provider may announce a /20 and hand you a /28 from it; only the /20 is visible elsewhere.

Can you advertise a /25 in BGP?

Rarely. The convention is to filter IPv4 prefixes longer than /24, so a /25 is dropped by most networks. To be routed independently you need a /24 or shorter and your own AS number.