What is an ASN?
An Autonomous System Number (ASN) is a unique number assigned to an autonomous system (AS) by the Internet Assigned Numbers Authority (IANA). An autonomous system is a collection of connected IP routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the internet.
How ASNs Work
ASNs are used in the Border Gateway Protocol (BGP) to uniquely identify each network on the internet. When routers exchange routing information, they use ASNs to determine the path data should take across the internet.
ASNs can be either:
- 16-bit — ranging from 1 to 65,534 (original format)
- 32-bit — ranging from 1 to 4,294,967,294 (extended format, RFC 6793)
The format is typically written as "AS" followed by the number, e.g.:
| ASN | Organization | Description |
|---|---|---|
| AS13335 | Cloudflare | CDN and DDoS protection |
| AS15169 | Search, cloud, services | |
| AS16509 | Amazon | AWS cloud infrastructure |
| AS8075 | Microsoft | Azure and Microsoft services |
| AS4766 | Korea Telecom | Major Korean ISP |
ASN in Security
In cybersecurity and reconnaissance, ASNs are valuable for:
- Asset discovery — finding all IP ranges belonging to an organization
- Attribution — identifying the network operator behind an IP address
- Threat intelligence — tracking malicious activity by network origin
- Infrastructure mapping — understanding hosting providers and CDNs
Search on Zondex
Use the asn: filter to search by ASN number:
asn:13335— find all hosts in Cloudflare's networkasn:15169— find all hosts in Google's networkorg:"Amazon"— search by organization name instead of ASN number