What an ASN is
An Autonomous System Number (ASN) identifies an autonomous system, which is a collection of IP address ranges managed under one routing policy. Internet providers, large companies, universities, and cloud platforms each operate one or more autonomous systems, and the ASN is how the global routing table refers to them.
How it is used
Networks exchange reachability information using the Border Gateway Protocol (BGP). ASNs sit at the center of this:
- Each network announces which IP blocks it controls, tagged with its ASN.
- BGP uses the chain of ASNs a route passes through to choose the best path.
- Regional registries assign ASNs so each one is globally unique.
ASNs come in 16-bit form, such as AS15169, and a newer 32-bit form for the larger pool.
Why it matters
The ASN reveals who actually operates the network behind an IP address, which is useful for troubleshooting, abuse handling, and understanding internet topology. For example, an IP that belongs to AS15169 is part of Google's network, while a residential address may belong to your internet provider's ASN. Looking up an address shows its owning ASN, the announced ranges, and the organization name. You can find the ASN behind any address with the WhatIP ip-lookup tool.