When you connect to the internet, your traffic doesn't travel in one hop from your device to its destination. It passes through multiple networks, each managed by different organizations. An Autonomous System Number (ASN) uniquely identifies each of these networks.

What Is an Autonomous System?

An Autonomous System (AS) is a collection of IP addresses under the control of a single organization — like an ISP, a large company, a university, or a cloud provider — with a consistent routing policy.

Examples of large autonomous systems:

  • Google: AS15169
  • Cloudflare: AS13335
  • Amazon (AWS): AS16509
  • Deutsche Telekom: AS3320

How BGP Uses ASNs

The internet's routing system — BGP (Border Gateway Protocol) — uses ASNs to determine how traffic flows between networks. Each AS announces which IP address ranges it controls, and BGP routers use this information to find optimal paths across the internet.

Without ASNs and BGP, traffic wouldn't know how to reach its destination across different networks.

What Does an ASN Lookup Reveal?

When you look up an IP address's ASN, you can find:

  • Which organization controls that IP
  • The ISP or hosting provider behind an IP
  • Whether an IP belongs to a data center, residential ISP, or VPN provider
  • The country and network the IP is registered in

This is why websites can detect VPN usage — VPN IPs often belong to ASNs registered to data centers rather than residential ISPs.

ASN vs IP Address

An IP address identifies a specific device or server. An ASN identifies the network that owns a range of IP addresses. One AS can control millions of IP addresses.

Private vs. Public ASNs

  • Public ASNs: Used for routing on the public internet. Originally 16-bit (1–65535), now also 32-bit (up to ~4.2 billion)
  • Private ASNs: Used internally (64512–65534 for 16-bit). Not advertised on the public internet

Look Up Any IP's ASN

Our IP lookup tool shows the ASN for any IP address — including your own. Enter any IP to see which organization controls it and what ISP it belongs to.

Conclusion

ASNs are the building blocks of internet routing. Every IP address belongs to an AS, and BGP uses ASNs to route traffic across the global internet. Understanding ASNs explains why VPN detection works, how traffic is routed, and who controls which parts of the network.