Question 1

What is an ASN?

Accepted Answer

An ASN (Autonomous System Number) is a unique identifier assigned to networks (ISPs, cloud providers, enterprises) that control IP address routing on the internet. ASNs are used in BGP (Border Gateway Protocol) routing to exchange routing information between networks. Examples: AS13335 (Cloudflare), AS15169 (Google), AS16509 (Amazon AWS).

Question 2

What can I learn from an ASN lookup?

Accepted Answer

An ASN lookup reveals the organisation that controls the network, the IP ranges (prefixes) announced by that ASN, the country of registration, the RIR (ARIN, RIPE, APNIC, etc.), and which ISPs peer with it. This is useful for security research, network troubleshooting, and understanding traffic routing.

Question 3

How do I find the ASN for an IP address?

Accepted Answer

Enter the IP address in the lookup tool — we will resolve it to its ASN automatically. You can also use the BGP routing table, WHOIS databases, or DNS-based lookups (e.g., origin.asn.cymru.com queries) to find the ASN for any IP.

Question 4

Why do security tools care about ASNs?

Accepted Answer

ASNs are used in threat intelligence to block or allow traffic from entire network ranges. Many organisations block ASNs known for hosting malicious traffic, bulletproof hosting, or Tor exit nodes. Conversely, allowing specific ASNs (e.g., Google, Cloudflare) can be used for allowlisting trusted crawlers and services.

ASN Lookup

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