What is an ASN (autonomous system number)?

An ASN, or autonomous system number, is the number that identifies one network on the internet. An autonomous system is a group of IP address blocks run by one or more operators under a single routing policy (RFC 1930). Networks use these numbers to tell each other which addresses they can reach. An IP lookup can therefore name the network that announces an address, and the owner of that network.

#How does an autonomous system work?

1. An operator, such as an internet provider, a cloud or a university, applies to its regional registry for an AS number.
2. The operator announces its address blocks to neighboring networks with BGP, the routing protocol between autonomous systems.
3. Each neighbor passes the route on with the list of AS numbers it crossed, called the AS path (RFC 4271), until the rest of the internet knows where those addresses live.
4. Traffic to any address in those blocks then flows to that network.

ARIN, the registry for North America, names three kinds of AS. A multi-homed AS connects to two or more others, so it stays online if one link fails. A stub AS connects to only one. A transit AS carries traffic between other networks, which is how providers connect their customers to the rest of the internet.

#How many ASNs are there?

The five regional registries had assigned 122,955 AS numbers by October 22, 2026, according to their combined statistics. Europe's registry, RIPE NCC, holds the most, and AFRINIC, the registry for Africa, the fewest.

AS numbers began as 16-bit values, which allow 65,536 numbers. The IETF started the move to 32-bit numbers in 2007, which allow 4,294,967,296, and RFC 6793 is the current standard for them. Among assigned numbers, the 32-bit kind now leads, at 66,421 against 56,534 (registry statistics, October 22, 2026).

The private ranges come from RFC 6996 and are meant for use inside private networks. ARIN now asks that every ASN be treated as a 4-byte number, whatever its size.

#How do I find the ASN of an IP address?

1. Paste the address into an ASN lookup or an IP lookup. Both show the network that announces it.
2. To see who registered a number, ask the registry's RDAP service. ARIN's answer for AS15169 names GOOGLE, registered on March 30, 2000.
3. In code, the ASN comes with every IP answer from an intelligence API.

curl https://api.ffraud.com/public/ip/8.8.8.8

Two public addresses show how different the networks behind an ASN can be. The answers come from FFraud's API on October 23, 2026. Where a count of flagged addresses appears, it covers only the addresses of that network FFraud has seen, so it describes observed traffic and makes no judgment of the operator.

Each network has its own page, such as AS15169 and AS60729.

#Why does a network's ASN matter for fraud checks?

The ASN tells you what kind of operator stands behind an address. Traffic from a cloud network to a consumer signup form can point to automation, while a mobile carrier's address is usually shared by many people behind carrier-grade NAT (what is CGNAT). Neither proves anything alone, so a network's type sets the context for the rest of the record.

Some small networks are almost entirely abusive. FFraud's open data names 561 small and medium networks where at least 90.1 percent of the addresses it observed were abusive and at least 70 percent were proxies or VPNs (build of October 23, 2026, 07:30 UTC). They range from 200 to 19,655 observed addresses, with a median of 486. By FFraud's published rule, large providers stay out of that file.

#What is an ASN confused with?

ASN also means other things outside networking, such as an advance shipping notice or a nursing degree. On an IP lookup, it always means the network number. To judge a single address rather than its network, read its IP reputation.

#Questions people ask

To see the network behind any address, and how its observed traffic looks, use the free ASN lookup.