What is an ASN? Autonomous system numbers explained, and how to find the ASN of an IP

An ASN, or autonomous system number, is the number that identifies a network on the internet’s routing table: an internet provider, a mobile carrier, a cloud or hosting company, a university or a large company that announces its own blocks of IP addresses to the rest of the internet. This is the networking ASN, not the advance shipping notice or the nursing degree. Every public IP address is announced by an autonomous system, so the ASN tells you which network an address belongs to. It does not tell you who is using the address. The facts below were checked on 18 October 2026.

What an autonomous system is

The internet is a network of networks, and each of those networks decides for itself how traffic enters and leaves it. The rulebook for that, RFC 1930 from 1996, defines an autonomous system as “a connected group of one or more IP prefixes run by one or more network operators” which has “a SINGLE and CLEARLY DEFINED routing policy”. The RIPE NCC, the registry for Europe, the Middle East and parts of Central Asia, puts it the same way on its page about AS numbers.

Autonomous systems tell each other which addresses they can reach using BGP, the Border Gateway Protocol (RFC 4271). When Google’s network announces the block 8.8.8.0/24, it is saying “send traffic for these addresses to AS15169”, and every network that hears the announcement passes it on with its own number added to the path. The last number in that path, the network that first announced the block, is the block’s origin AS, and that is the ASN people mean when they ask for the ASN of an IP address.

There are a lot of them. The CIDR Report for 18 October 2026 counted 79,681 autonomous systems in the routing system, 28,161 of which announced a single prefix.

ASN examples: who announces what

An AS has a number and a short registry name chosen by its operator, often a handle rather than the company’s full name. Here are five well-known ones, as the routing data behind Maskbreak’s ASN lookup showed them on 18 October 2026:

Two things the table shows. One company can run several autonomous systems: Amazon’s cloud space is spread over AMAZON-02 and AMAZON-AES, among others, and Google’s cloud has its own, AS396982 (GOOGLE-CLOUD-PLATFORM, 22,014,976 IPv4 addresses that day), so “Google’s ASN” has more than one answer. And size in addresses says little about importance: Cloudflare announces fewer IPv4 addresses than DigitalOcean because it serves many websites from shared addresses, while a cloud host gives each rented server an address of its own.

16-bit and 32-bit ASNs, and the reserved numbers

AS numbers were originally 16-bit, from 0 to 65535. To prepare “for the anticipated exhaustion of the two-octet AS numbers”, RFC 6793 (December 2012) expanded the pool “from 0-65535 to 0-4294967295”. Routers too old to understand the longer numbers see AS 23456, called AS_TRANS, in their place.

The numbers are written as plain decimals, such as AS396982, Google Cloud’s 32-bit number. RFC 5396 calls that “asplain” and proposes it for all documents, systems and user interfaces; an older “asdot” style wrote numbers above 65535 as two parts joined by a dot, so 65546 became 1.10, and you may still meet it in old router configurations.

Some numbers are never handed out. The IANA registry of AS numbers (last updated 1 June 2026) reserves these:

Private-use numbers work like private IP addresses: a network can use them inside itself, for example between the switches of a data centre, but they are not unique, and RFC 6996 requires them to be removed from a route before it is announced to the global internet.

Everything else is handed out by IANA in blocks to the five regional internet registries, AFRINIC, APNIC, ARIN, LACNIC and the RIPE NCC, which assign numbers to the networks in their regions.

Who gets an ASN

The rules differ by registry, and they have loosened over the years. RFC 1930 said that a site connected to more than one provider, which it called multihomed, should normally have its own AS, and that this “is ALMOST THE ONLY case where a network operator should create its own AS number”. Today:

• ARIN (North America) says “Any organization may be issued a single Autonomous System Number (ASN) upon request”, and asks for proof of a unique routing policy before issuing more (ARIN Number Resource Policy Manual, version 2025.1 of 3 March 2026).
• The RIPE NCC says “Current RIPE Policy requires a network to be multi-homed, and have a unique routing policy for an ASN to be assigned” (RIPE NCC). A policy proposal, 2026-01, would let holders of RIPE NCC address space get their first ASN without that justification; on 18 October 2026 it was in its review phase, at version 2.0 of 12 October.

Most organisations never need one. A shop, a small company or a household uses its provider’s network, and its addresses are announced under the provider’s ASN.

How to find the ASN of an IP address

In a browser. Type the address into Maskbreak’s ASN lookup. On 18 October 2026 it answered “8.8.8.8 is announced by AS15169 (GOOGLE), registered in US”, with links to the AS page and its announced ranges. It takes IPv4 and IPv6 addresses, AS numbers and network names, is free and needs no account.

From a terminal. Team Cymru runs a free IP-to-ASN mapping service over whois and DNS. The whois form needs a space before the flags, inside the quotes:

whois -h whois.cymru.com " -v 8.8.8.8"

AS      | IP               | BGP Prefix          | CC | Registry | Allocated  | AS Name
15169   | 8.8.8.8          | 8.8.8.0/24          | US | arin     | 2023-12-28 | GOOGLE - Google LLC, US

The DNS form writes the address backwards (1.2.3.4 becomes 4.3.2.1) in front of origin.asn.cymru.com, which is convenient for scripts; Team Cymru asks for the DNS interface for regular bulk queries:

dig +short 1.1.1.1.origin.asn.cymru.com TXT
"13335 | 1.1.1.0/24 | AU | apnic | 2011-08-11"

dig +short AS13335.asn.cymru.com TXT
"13335 | US | arin | 2010-07-14 | CLOUDFLARENET - Cloudflare, Inc., US"

Read the country column with care. In that answer the block 1.1.1.0/24 is listed under APNIC with the country AU, while the AS that announces it, Cloudflare’s AS13335, is registered with ARIN in the US. Neither is where the person using the address is. Team Cymru says plainly that its service is not a GeoIP service.

What an ASN tells you about a visitor, and what it does not

Fraud and security teams look up the ASN because it says what kind of network a visit came from: a home broadband provider, a mobile carrier, a cloud host. A signup from DigitalOcean’s or Amazon’s network is unusual for a person at home, so the ASN is useful context.

It is not a verdict. VPN services rent servers from hosting companies, residential proxies run on ordinary home connections (what a residential proxy is), and companies send staff browsing through cloud gateways, so the same AS carries real customers and hidden ones alike. The guide to datacenter IP detection goes through the cases where a cloud address is perfectly legitimate.

That is why Maskbreak keeps routing data out of its risk score. Its lookup shows the AS that announces an address as context; the cloud-server flag on an answer comes from the network reading and the address ranges nine cloud providers publish (listed on the hosting ranges page), and its VPN and proxy verdicts come from a live visit to your page, not from the AS name.