ASN Lookup
This free ASN lookup shows who runs an autonomous system. Enter an AS number like AS13335, an IP address to find the network that announces it, or a network name to search, and see the holder, the IPv4 and IPv6 prefixes it announces, its top upstream and downstream neighbours and its registration record.
The lookup runs in your browser, so we never see what you type and nothing is stored. Your browser contacts Cloudflare (cloudflare-dns.com) or Google (dns.google) for DNS answers, including Team Cymru's address-to-network data, stat.ripe.net (RIPEstat) for the holder, prefixes and neighbours, and the registries' RDAP servers (found through data.iana.org) for registration data.
Free, no signup.
What an ASN lookup shows
An autonomous system (AS) is a network run by one organisation, such as a hosting company, an internet provider or a large website, and it is identified by an AS number like AS13335. The overview names the holder, says whether the network is announced on the internet right now, and shows the country, registry and date from Team Cymru's data.
Below the overview you get the announced prefixes in separate IPv4 and IPv6 tabs, with a filter and a Copy all button, the strongest upstream and downstream neighbours, and the registration record with its organisation and abuse contact.
Three ways to search
Enter an AS number, with or without the AS in front, to see that network. Enter an IPv4 or IPv6 address to find which network announces it: the page looks up the origin AS and then shows that network. Enter a name of two or more letters, such as cloudflare, and you get a list of matching networks to click.
The address in your browser's address bar is updated with your search, so you can copy it and share the result.
Prefixes, upstreams and downstreams
A prefix is a block of IP addresses, written like 1.1.1.0/24, that an AS tells the rest of the internet it can deliver traffic to. Big networks announce thousands of them, so the page shows the first 500 for each family and tells you how many there are in total. The filter narrows the list, and Copy all copies every prefix.
Upstream neighbours are the networks an AS gets its connection to the rest of the internet from, and downstream neighbours are the networks that connect through it. The page lists the 15 strongest of each, ranked by the power value from RIPEstat, which grows with how often the pair appears together in routing data.
Where the data comes from
Your browser asks the sources directly, and we do not see what you look up. The holder, the prefixes, the neighbours and the name search come from RIPEstat (stat.ripe.net). The country, registry and allocation date, and the origin AS for an IP address, come from Team Cymru's service, read through DNS over HTTPS from Cloudflare (cloudflare-dns.com) with Google (dns.google) as a fallback.
The registration record comes from the RDAP server of the regional internet registry that issued the AS number, found through the lists at data.iana.org. If one of these sources is busy or down, only its card shows a message and the other cards still fill in.
What to do with the result
Use the abuse contact when you need to report spam or attacks coming from a network. Use the prefix list to build firewall or allow lists, or to check whether an address range belongs to the network you expect. Use the neighbours to see which providers a network depends on.
If you run your own server, the IP Lookup and Reverse DNS Lookup tools are good next steps for a single address, and the blacklist check shows whether it is listed anywhere.
Frequently asked questions
What is an ASN?
An ASN (autonomous system number) identifies a network that is run by one organisation and has its own routing policy on the internet. Hosting companies, internet providers, universities and large websites each have one or more, written like AS13335.
How do I find the ASN of an IP address?
Enter the address in the box. The page asks Team Cymru's IP-to-ASN service which AS announces the block the address belongs to, and then shows that network with its prefixes, neighbours and registration record.
What is an announced prefix?
It is a block of IP addresses, such as 1.1.1.0/24 or 2606:4700::/32, that a network tells the rest of the internet it can deliver traffic to. If a prefix is not announced, nobody can route traffic to it.
What are upstream and downstream networks?
An upstream is a network that gives an AS its path to the rest of the internet, like a transit provider. A downstream is a network that connects through this AS. The page ranks both by a power value taken from routing data, so the strongest links come first.
Why does the page show only 500 prefixes?
Some networks announce thousands of prefixes, and drawing them all would make the page slow. It shows the first 500 for IPv4 and for IPv6 and says how many there are. The filter narrows the list, and Copy all copies the full list.
Why does a card say it couldn't load?
The data comes from several public services, and any of them can be busy, slow or temporarily down. Each card loads on its own, so one failing does not stop the others. Try the lookup again after a moment.
Who do I contact about abuse from a network?
Use the abuse contact in the registration card. It is the email address the network's owner publishes for reports of spam, attacks and other misuse. Include the date, the time and the log lines that show the problem.
Do you store the networks I look up?
No. The lookup runs in your browser and we do not receive what you enter. Your browser asks Cloudflare or Google for DNS answers, stat.ripe.net for the network data and the registries' RDAP servers for the registration record, so those services can see the request.
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