How to use it
- 1.Enter an AS number such as
AS13335(theASis optional), or part of a network’s name such asCloudflare. - 2.Click Look up. A name search lists the matching networks; pick one to see its details.
- 3.Read the summary: who holds the AS, where it is registered and how many IPv4 and IPv6 prefixes it announces.
- 4.Open Prefixes to see every announced range, copy them all at once or rest on one for its route details. Open Neighbours to see the networks it connects to.
What an autonomous system is
An autonomous system (AS) is a network run under one routing policy: an internet provider, a cloud or hosting company, a university or a large business. Autonomous systems exchange routes with each other over BGP, and together they make up the internet.
Each one is identified by an AS number (ASN) that a regional internet registry (RIPE NCC, ARIN, APNIC, LACNIC or AFRINIC) assigns from blocks it gets from IANA. The original numbers have 16 bits (up to 65535); since 2007 registries also hand out 32-bit numbers, which now make up most new assignments.
An AS announces the IP prefixes it is responsible for, and every route on the internet carries an AS path: the chain of autonomous systems a packet crosses to reach that prefix. That makes the AS number the quickest way to see which address ranges a network uses.
Where the data comes from
Announced prefixes and neighbours come from RIPE RIS, the RIPE NCC’s Routing Information Service. Its route collectors receive BGP feeds from hundreds of networks around the world, and RIPEstat publishes what they see. The prefix list shows what was announced at the latest snapshot, which RIPEstat refreshes several times a day.
Names are taken from bgp.tools, which publishes the name of every AS once a day. The holder, country, registration date and abuse contact come from the registry’s own RDAP record, the same source as our WHOIS lookup.
Route collectors only see what their peers pass on. A prefix used inside a network, announced to a single partner or filtered on the way doesn’t show up, so an empty list doesn’t always mean the AS is unused.
Upstream and downstream neighbours
Neighbours are the autonomous systems that appear right next to this one in AS paths. A network seen upstream sits on the side of the collector, towards the rest of the internet; that’s usually a transit provider. A network seen downstream sits on the side of the origin; that’s usually a customer whose routes this AS carries.
The paths count says how often the two appear next to each other, which shows how important the link is. It isn’t a statement about contracts: a peer at an internet exchange can show up on either side, and links that collectors don’t hear about stay invisible. Networks that appear on both sides are listed separately.
Questions and answers
How do I find the AS number of an IP address?
Enter the address in the IP lookup: it shows the AS that announces the address and the prefix it belongs to. From there, rest on the AS number or click it to come back here with all of that network’s prefixes.
How do I get all IP ranges a company uses?
Search for the company’s name, pick its AS and open Prefixes, then click Copy all prefixes for a plain list with one prefix per line. Large companies often run several AS numbers (for different regions or acquisitions), and some of their addresses may be announced by their providers instead, so check every match.
What is the difference between 16-bit and 32-bit AS numbers?
16-bit numbers run from 1 to 65535 and ran short in the 2000s, so 32-bit numbers (up to 4294967295) were introduced. Both work the same way in BGP. Routers too old to understand 32-bit numbers see the placeholder AS23456 (AS_TRANS) instead.
Why does an AS announce no prefixes?
The number may be assigned but not in use yet, used only for private connections, or kept for a network that is being built. Private numbers (64512–65534 and 4200000000–4294967294) never appear on the internet at all.
Are the networks seen upstream really its transit providers?
Usually, but not always. The lists come from AS paths, which show who passes routes to whom, not the contracts behind them. A peer at an internet exchange can appear on either side, and connections the route collectors don’t hear about are missing. Use the paths count as a sign of how significant a link is.