What is an ASN?
An ASN (autonomous system number) is a network's ID on the internet. Carriers, clouds, universities and large companies that manage their own routing each have one or more. AS4134, for example, is China Telecom's 163 backbone; AS13335 is Cloudflare.
Each AS announces which blocks of IP addresses (prefixes) it handles, and the rest of the internet uses that to decide where to send traffic. The protocol behind this is BGP. AS numbers are handed out by five regional registries: APNIC (Asia-Pacific), RIPE NCC (Europe and the Middle East), ARIN (North America), LACNIC (Latin America) and AFRINIC (Africa).
How do I find who owns an IP?
Type the IP in the box. You'll see which AS, meaning which network, it's on; which prefix it's in; and the registry record for that block: network name, who it's registered to, the abuse contact and reverse DNS.
The network that routes an address and the organization it's registered to are often different. A company running servers in the cloud, for example, may have its block announced by the cloud provider. To check whether an IP is a data center IP or home broadband, and whether it's been flagged for abuse, open “Is this IP clean?” under the result.
Is the country shown where the IP actually is?
Not necessarily. It's the country the network is registered in, not where this IP is.
Clouds and CDNs have data centers all over the world: a network registered in the US can easily have IPs in Hong Kong or Singapore.
Where does the data come from, and how fresh is it?
Prefixes come from RIPE NCC's Routing Information Service, which collects routing tables from hundreds of networks; a prefix is listed when at least ten of them see it. Which AS an IP is on comes from iptoasn.com (public domain). Names, countries and assignment dates come from the five registries' published files. All of these are updated every day.
The registration, abuse contact, reverse DNS and connected networks are looked up live (RDAP, DNS, RIPEstat) when you open a page.
What do upstream and downstream mean?
An upstream is a network this AS pays to reach the rest of the internet. A downstream is a network that reaches the internet through it, such as its customers.
These links are worked out from the paths RIPE's collectors see. Networks that show up on only some paths, usually peers that swap traffic directly, are listed under “Peers and others”. Each column is ranked by how many paths go through each network, so the main connections come first.
What are CN2, 9929 and CMI?
They're the premium international networks of China's three big carriers, often advertised for VPS routes to China: AS4809 is China Telecom's CN2 (the regular backbone is AS4134, “163”), AS9929 is China Unicom's premium network (the regular one is AS4837, “169”), and AS58453 is China Mobile International (the domestic network is AS9808).
Which one a server's traffic takes depends on the route back to you. A traceroute shows the AS numbers along the way.
Can I copy the prefix list?
Yes. “Copy all” copies every IPv4 or IPv6 prefix of the AS, one per line, ready to paste into a firewall or proxy rule list. For scripts, /api/as/<asn> returns the full data for any AS as JSON.
