What Is an ASN (Autonomous System Number)?

An ASN is the number behind a network on the internet, like AS15169 for Google. How BGP links them, who assigns ASNs, 16-bit vs 32-bit and how to look one up.

What Is an Autonomous System?

An Autonomous System (AS) is a network, or group of networks, run by one organization under one routing policy, and the ASN is its unique number. The internet is a network of these networks: every big ISP, cloud provider, university and large company runs its own AS, and they swap routing information so a packet can get from any of them to any other. Some you will recognize:

AS15169: Google
AS13335: Cloudflare
AS16509: Amazon (AWS)
AS8075: Microsoft
AS32934: Facebook (Meta)

Scale: as of 2025 there are over 75,000 active ASNs, each with its own IP blocks and routing policy. Every IP address you will ever see belongs to one of them.

BGP: How Autonomous Systems Communicate

BGP (Border Gateway Protocol) is how autonomous systems tell each other which routes they have. It decides the path data takes between networks, which is why it gets called the routing protocol of the internet.

Each AS announces the IP prefixes (address blocks) it owns to its neighbors.
The neighbors pass those announcements on, and the global routing table builds up from that.
For a packet going from AS A to AS B, BGP picks the best path through the ASes in between, based on policy, path length and other attributes.

ASes relate to each other in a few ways:

Transit. A customer AS pays a provider AS to carry its traffic to the rest of the internet.
Peering. Two ASes swap traffic directly, often for free, usually at an Internet Exchange Point (IXP). Lower latency, lower transit bills.
Upstream and downstream. The AS providing connectivity is upstream; the one receiving it is downstream.

Who Assigns ASNs?

The same organizations that hand out IP addresses:

Organization	Role	Region
IANA	Global ASN pool management	Worldwide
ARIN	Regional allocation	North America
RIPE NCC	Regional allocation	Europe, Middle East, Central Asia
APNIC	Regional allocation	Asia-Pacific
LACNIC	Regional allocation	Latin America, Caribbean
AFRINIC	Regional allocation	Africa

To get one, an organization has to show it needs its own routing policy and peers, or plans to peer, with more than one upstream provider.

16-Bit vs 32-Bit ASNs

ASNs started as 16-bit numbers, 0 to 65,535. Those ran out, so 32-bit ASNs were added:

Type	Range	Total Available	Format
16-bit ASN	1 to 65,534	~65,000	`AS15169`
32-bit ASN	65,536 to 4,294,967,294	~4.2 billion	`AS394842`

Reserved ranges:

AS0: reserved, never used
AS64496 to 64511: documentation and examples
AS64512 to 65534: private use, like private IP addresses
AS23456: "AS_TRANS", a placeholder for backward compatibility during the 16-to-32-bit transition

Looking Up ASN Information

Our tool: ASN Lookup shows the organization, country and IP prefixes behind any ASN.
WHOIS: whois AS15169 queries the right RIR database.
BGP looking glasses: RIPE RIS, RouteViews and Hurricane Electric's BGP Toolkit show live routing data.
IP to ASN: whois -h whois.cymru.com " -v 8.8.8.8" tells you which AS an IP belongs to.

ASNs are how you trace a routing problem, work out which network is behind an IP in a security investigation, and see how the internet fits together.

ASN Lookup Tool
Try it on a real address. Free, no account needed.