DNS
IP-to-ASN and network lookup
Look up which autonomous system and network an IP address belongs to, and who that network is registered to.
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Result
The verdict appears here once you press Look up the ASN.
What this is
This ip to asn lookup reads Team Cymru's published mapping from an address to the autonomous system that announces it on the internet's own routing table, then names the organisation that network is registered to - the two facts a reseller actually needs before trusting a sending IP rather than just a number.
An autonomous system (AS) is one operator's own routing domain: a cloud provider, an ISP or a hosting company that announces a block of addresses to the rest of the internet under one AS number. Every public IPv4 or IPv6 address that is actually routed belongs to exactly one such announcement at any given moment, which is what this lookup reads rather than guessing from a WHOIS allocation record alone.
Team Cymru's mapping service answers this as an ordinary DNS TXT record, queried at a name built backwards out of the address under origin.asn.cymru.com - the same shape a PTR lookup uses, just under a different zone. A second, separate TXT query at AS<number>.asn.cymru.com turns that bare number into the registered holder's name.
How to read the result
The facts above are the whole answer: the ASN, the network block (BGP prefix) it was found inside, the country code and registry that allocated it, and the organisation name when Team Cymru has one on file for that AS number. A reseller checking a sending IP reads the holder name first - an address inside a named hosting or mail provider's own AS is a very different signal from the same address sitting inside a residential ISP's AS, even when both pass every other check.
No autonomous system published for an address is common and not alarming on its own: it is the expected answer for a private or documentation-range address, and it also happens for a public address that nobody currently announces on the routing table - which this page explains rather than treating as a broken lookup.
Common mistakes
What goes wrong most often, and how to tell.
- Treating the ASN alone as meaningful without its holder nameAS15169 says nothing to a person reading it until it is matched to the name the registry has on file. The number is stable and worth recording, but the holder name is what actually tells a reseller whether a sending IP sits inside a known provider or somewhere unexpected.
- Expecting an ASN for a private or reserved addressAn address inside a private, loopback or documentation range was never announced on the public routing table in the first place, so an empty answer there reflects what the address is for, not a fault with Team Cymru's mapping or with this lookup.
- Assuming one address inside a network is who the whole network says it isAn AS holder name describes the operator of the network, not the specific sender using one address inside it. A legitimate business can rent space inside a network whose holder name is a generic hosting company, and a bad actor can briefly appear inside one with a trusted name too.
Questions
What people ask about this tool.
- It names the one network operator who currently announces that address to the rest of the internet, which is the first fact worth checking before trusting any address: the same block inside a known business hosting network reads very differently to a receiver than the same block inside a residential ISP, even before anything else about the traffic is examined.
- That is the expected answer for a private, loopback or documentation-range address, which was never announced on the public routing table to begin with, and it also happens for a public address that is not currently being routed by anyone - neither case is a fault with this lookup or with Team Cymru's mapping service.
- It is a separate lookup at AS<number>.asn.cymru.com, which Team Cymru maps from the regional internet registries' own records for that AS number. It names the organisation that registered and operates the network, not the specific customer sending from one address inside it.
- Yes - a prefix that is multi-homed across more than one network operator shows up as several AS numbers in the same answer, which this lookup calls out rather than silently picking one and discarding the rest.
Sources
Every fixed value here comes from the provider’s own documentation. These pages change, so each says the day it was last read.
- Team Cymru, “IP to ASN Mapping” read 13 October 2026
Use it from the API
The same check, as one request.