Find an AS Number: A Simple Guide

G5 Cyber Security

2 weeks ago

TL;DR

Need to find out who owns a specific Autonomous System (AS) number? This guide shows you several free and easy ways to do it, from using online tools to checking public databases. It’s useful for network troubleshooting, security investigations, or just understanding internet routing.

How to Find an AS Number

Understand What an AS Number Is

An AS number is a unique identifier assigned to an autonomous system – typically a large network (like an Internet Service Provider or big company) that controls its own routing policies. It’s essential for internet traffic exchange.

Use Online AS Number Lookup Tools

Team Cymru’s ASN Lookup: This is a very reliable and quick tool.
```
https://bgpview.io/asn/{AS_NUMBER}
```
Replace {AS_NUMBER} with the number you’re looking for (e.g., https://bgpview.io/asn/6939).
Hurricane Electric’s BGP Toolkit: Another excellent resource.
```
https://bgptoolkit.hurricaneelectric.com/
```
Enter the AS number into the search box.
IPinfo.io: Offers a free ASN lookup alongside IP address information.
```
https://ipinfo.io/asn/{AS_NUMBER}
```

Check Public BGP Databases (RIRs)

Regional Internet Registries (RIRs) maintain databases of AS number allocations. These are the official sources.
- ARIN: American Registry for Internet Numbers (https://www.arin.net/). Covers North America.
- RIPE NCC: Réseaux IP Européens Network Coordination Centre (https://www.ripe.net/). Covers Europe, the Middle East and parts of Central Asia.
- APNIC: Asia-Pacific Network Information Centre (https://www.apnic.net/). Covers Asia Pacific region.
- LACNIC: Latin American and Caribbean Internet Addresses Registry (https://www.lacnic.net/). Covers Latin America and the Caribbean.
- AfriNIC: African Network Information Centre (https://www.afrinic.net/). Covers Africa.
You’ll usually need to search their databases using the AS number. The websites have search functions – look for ‘ASN Lookup’ or similar.

Use Command-Line Tools (for more technical users)

Whois: You can use the whois command to query RIR databases directly.
```
whois -h whois.arin.net {AS_NUMBER}
```
Replace {AS_NUMBER} with the AS number you want to look up. You may need to change the host (-h) depending on which RIR holds the information (e.g., whois.ripe.net).
BGP Looking Glass: Some ISPs provide a ‘looking glass’ service, allowing you to view BGP routing information from their network.
```
traceroute -T {AS_NUMBER}
```
This can sometimes reveal the AS path. Note this requires understanding of traceroute and networking concepts.

Interpreting the Results

The results will typically show you:
AS Number: The unique identifier itself.
Organization Name: The name of the network operator owning the AS number.
Contact Information: Details for contacting the network administrator (often technical and abuse contacts).
IP Address Ranges: The blocks of IP addresses associated with that AS number.