What is an ASN (Autonomous System Number)?

What is an Autonomous System?

Before you can understand an ASN, you need the thing it numbers: the Autonomous System, usually shortened to AS. An Autonomous System is a network — or, more precisely, a collection of IP address blocks (called prefixes) — that is operated under a single, clearly defined routing policy. In plain terms, it is one administrative domain that decides, as a unit, how its addresses are announced to the rest of the internet and how it chooses to send traffic outward.

The organisation behind an AS is almost always something substantial: an internet service provider, a cloud or hosting provider, a large university, a government department, or a big company that runs its own infrastructure. A home network is not an AS — it borrows a public IP from its ISP, which sits inside the ISP's Autonomous System. But the ISP itself is an AS, because it owns ranges of addresses and needs to tell the rest of the world, “these prefixes are reachable through me.”

The ASN — Autonomous System Number — is simply the globally unique identifier attached to that AS. It is the name tag the internet's routing system uses so that every other network can refer to it without ambiguity.

How ASNs are written

An ASN is written as the letters AS followed by a number, with no space in between. The number is unique across the entire internet, so a single ASN always points to exactly one network. A few well-known examples make the pattern concrete:

• AS13335 — Cloudflare, the network behind a large share of the web's CDN and DNS traffic.
• AS15169 — Google, covering much of its search, cloud, and consumer infrastructure.

When you look up an IP address and see it “belongs to AS15169,” that is telling you the address is announced to the world by Google's Autonomous System. The ASN is the link between an individual address and the organisation that operates the network it lives in.

16-bit vs 32-bit ASNs

ASNs were originally defined as 16-bit numbers. That gives a range of 0 to 65535 — 65,536 possible values in total. In the early internet that felt like more than enough, but as the number of independent networks grew, the 16-bit pool started running low, in much the same way IPv4 addresses did.

The fix was to extend the format to 32-bit ASNs, raising the ceiling to 4294967295. Crucially, this was not a separate, incompatible scheme: the old 16-bit numbers are just the low end of the same continuous 32-bit space. A modern router treats a small number like AS13335 and a large 32-bit number such as AS394089 in exactly the same way. If you see an ASN above 65535, you are simply looking at a network that was assigned a number from the extended range.

A handful of ASN values are set aside for special purposes rather than handed out to public networks. There are reserved and documentation ASNs, and there are private ASN ranges — analogous to the private IP ranges covered in public vs private IP addresses — that organisations can use internally without them ever appearing on the public internet. You will not see a private ASN when you look up a public IP, because by design those numbers never leak into the global routing table.

How ASNs and BGP work together

An ASN would be a meaningless label if there were no system that used it. That system is BGP, the Border Gateway Protocol — the protocol that stitches all the world's Autonomous Systems into one internet. BGP is how networks tell each other which addresses they can reach and how.

Every AS uses BGP to announce the prefixes it owns: “I am AS15169, and I can deliver traffic for these address ranges.” Neighbouring networks receive that announcement, tag it with the announcing ASN, and pass it along to their own neighbours. As an announcement travels across the internet it accumulates a list of the ASNs it has passed through — the AS path — which routers use to choose the best route and to avoid loops.

Networks connect to each other in two broad ways. In peering, two ASes agree to exchange traffic between their own customers directly, often at no cost. In transit, one AS pays another to carry its traffic onward to the rest of the internet. Either way, the ASN is the unit of identity: peering and transit agreements are made between ASNs, and the routing table is fundamentally a map of which prefixes each ASN announces.

Because the whole system runs on trust in those announcements, it can be abused. If a network announces prefixes it does not actually own, it can hijack traffic destined for someone else — the subject of BGP hijacking. The ASN is central to detecting this: when the AS announcing a prefix suddenly changes to one that has no legitimate claim to it, that is the fingerprint of a hijack.

Who assigns ASNs

ASNs are handed out through the same hierarchy that governs IP address allocation. At the top sits IANA, the Internet Assigned Numbers Authority, which holds the global pool of ASNs. IANA does not deal directly with individual networks. Instead, it delegates large blocks of numbers to the five Regional Internet Registries (RIRs), each responsible for one part of the world:

• ARIN — the United States, Canada, and parts of the Caribbean.
• RIPE NCC — Europe, the Middle East, and Central Asia.
• APNIC — the Asia-Pacific region.
• LACNIC — Latin America and the Caribbean.
• AFRINIC — Africa.

Each RIR then assigns individual ASNs to the networks in its region. To qualify, an organisation generally needs a genuine reason to run its own routing policy — typically because it connects to more than one upstream provider and wants to announce its own address space independently. This is why the RIR that assigned an ASN is itself a clue about roughly where a network is based.

Why ASNs matter in practice

For anyone investigating an IP address, the ASN is one of the most useful facts you can learn, because it answers the question “who is actually behind this address?”

• Identifying the owner and operator. An IP on its own is just a number. The ASN ties it to a named organisation, which is the starting point for any deeper lookup. Registration records exposed through WHOIS and RDAP tie an address and its ASN back to the organisation that holds them.
• Telling hosting apart from residential. The ASN usually reveals whether an address belongs to a consumer ISP, where real people browse, or to a cloud or hosting provider, where servers and automated traffic live. That distinction — explored in what is a hosting IP — matters a great deal when you are judging whether a visitor is a person or a machine.
• Reputation and abuse. Reputation systems often track behaviour at the level of the ASN, not just the single address. A network that consistently sources spam, scanning, or attacks earns a poor reputation for its whole ASN, which is why the IP reputation of an address is coloured by the company it keeps inside its Autonomous System.
• Understanding connectivity. Knowing an address's ASN, and the peering and transit relationships around it, tells you how well connected a network is and who it depends on to reach the rest of the internet — useful for everything from performance analysis to security research.

Frequently asked questions

What does an ASN actually identify?

An ASN identifies an Autonomous System — a network, or collection of IP prefixes, operated under a single routing policy, which almost always means one organisation such as an ISP, cloud provider, university, or large company. It is the globally unique number the internet uses to refer to that network when exchanging routes. For example, AS13335 is Cloudflare and AS15169 is Google.

What is the difference between a 16-bit and a 32-bit ASN?

The original ASN format was 16-bit, giving numbers from 0 to 65535. That pool was being exhausted, so the format was extended to 32-bit, raising the ceiling to 4294967295. A 16-bit ASN is just a 32-bit ASN with a small value — both share one continuous number space, so small and large ASNs are handled identically by modern routers.

Who assigns ASNs?

IANA holds the global pool and delegates blocks of ASNs to the five Regional Internet Registries: ARIN, RIPE NCC, APNIC, LACNIC, and AFRINIC. Each RIR then assigns individual ASNs to the networks in its region, typically ISPs and organisations that need to run their own routing policy.

Related reading

• BGP hijacking explained — how the routing system that gives ASNs their meaning can be abused, and how the ASN reveals it.
• WHOIS and RDAP lookup — see the registration records that tie an IP and its ASN to a named organisation.
• What is a hosting IP? — how the ASN helps tell datacenter and cloud addresses apart from residential ones.
• IP reputation and blocklists — why reputation is often tracked at the level of the whole ASN.
• Public vs private IP addresses — the same public/private split that ASNs also have, at the address level.
• The IPFerret glossary — short definitions of the other networking terms in this article.