What is an ASN (Autonomous System Number)?

An Autonomous System Number (ASN) is a globally unique identifier for an autonomous system — a network, or group of networks, operated under a single, clearly defined routing policy. If an IP address is a street address, an ASN is closer to the name of the entire postal company that delivers across a whole region. It is the unit the internet uses to route traffic between the thousands of independent networks that compose it.

Why ASNs exist

The internet is not one network. It is the voluntary interconnection of tens of thousands of separate networks — internet service providers, cloud platforms, universities, content companies, governments — that agree to exchange traffic. For any of them to send data to another, each needs a stable, unique identity. That identity is the ASN.

Consider what happens when you open a web page hosted in another country. Your request does not travel down a single wire. It crosses a sequence of autonomous systems: your ISP’s network, perhaps a regional transit provider, an internet exchange, and finally the network that hosts the site. Each of those is an autonomous system with its own number, and the route your traffic takes is described entirely in terms of those numbers.

Autonomous systems and BGP

The protocol that ties autonomous systems together is the Border Gateway Protocol (BGP) — the routing system of the entire internet. Each autonomous system uses BGP to announce the IP prefixes it is responsible for, tagged with its ASN, and to learn the prefixes every other network announces.

When a route is shared across the internet, it accumulates an AS path: the ordered list of autonomous systems it has crossed. Reading an AS path tells you the origin — the network that owns and announces the destination prefix — and every network that relayed the route along the way. This is, in effect, the internet’s chain of custody for reachability, and it is why a single ASN is so informative: it sits at a known position in a global graph of who connects to whom.

How an ASN is structured

ASNs are just numbers, but their range has a history worth knowing.

• 16-bit ASNs (0 to 65,535) were the original format. As the internet grew, this space began to run out.
• 32-bit ASNs (up to roughly 4.29 billion, i.e. 4,294,967,295) were introduced to extend the supply and are now allocated as a matter of course. Any number above 65,535 is simply a 32-bit ASN; modern routers handle both transparently.
• Certain ranges are reserved: AS0 is invalid as an origin, and private ASN ranges (64,512–65,534 for 16-bit, and 4,200,000,000–4,294,967,294 for 32-bit) are set aside for use inside a single organisation, never to appear on the public internet — the ASN equivalent of private IP addresses.

Who needs an ASN, and how to get one

Most networks do not need their own ASN. A home connection, or a business that buys its internet from a single provider, simply uses the addresses and routing of that provider. You need your own ASN when you are multihomed — connected to two or more other networks — and want to control how traffic enters and leaves your network independently of any single upstream.

To obtain one, an organisation applies to its Regional Internet Registry — RIPE NCC, ARIN, APNIC, LACNIC, or AFRINIC — usually as a member or through a sponsoring provider, demonstrating a genuine routing-policy need. The same registries allocate the IP address space, which is why address and ASN records line up so cleanly in a registry index like ipdex.

The relationships between networks

Autonomous systems do not connect at random; they form economic relationships that shape how traffic flows. The three classic types are:

• Transit (provider–customer): a customer network pays a provider to carry its traffic to the rest of the internet.
• Peering: two networks of comparable scale exchange traffic between their own customers directly, usually settlement-free, often at an internet exchange.
• Customer cone: the set of networks an operator can reach through its customers — a rough measure of how central it is to the internet’s structure.

This topology is real, measurable data. Research projects such as CAIDA infer these relationships from global routing, and ipdex surfaces them on each network’s profile — its providers, peers, and the size of its customer cone — turning a bare number into a map of the operator’s place in the internet.

What an ASN reveals

Because an ASN corresponds to a real, registered operator, it is one of the most useful facts you can derive from an IP address. From an ASN you can typically learn the operator’s name and the type of network — residential ISP, mobile carrier, hosting or cloud provider, content network, enterprise, or government. You can see the country of registration, the number of IPv4 and IPv6 prefixes it announces (a rough proxy for size), and, through routing data, its providers, peers, and customer cone.

The public DNS resolver 8.8.8.8, for instance, is operated by Google’s network, AS15169 (Google LLC) — a real autonomous system resolved from a single address. The live card at the end of this article shows that data directly from the ipdex index, with a link to the full profile. If a particular network is not in the index, the lookup reports an honest “not found” rather than guessing — accuracy over invention.

Myth-busting

Myth: “An ASN is the same as an IP address.” No. An IP address identifies one interface; an ASN identifies an entire network of many addresses under one routing policy. One ASN announces many IP prefixes.

Myth: “Every company on the internet has an ASN.” Most do not. Only networks that route independently — typically multihomed operators — need one. A company hosting its site on a cloud platform uses that platform’s ASN, not its own.

Myth: “A bigger ASN number means a bigger or older network.” The number is just an identifier handed out in sequence over time. A low number often means an early allocation, but it says nothing reliable about a network’s current size or importance — the routing data does.

Myth: “An organisation has exactly one ASN.” Large operators frequently hold many — for different regions, products, or acquired companies. This is exactly why ipdex groups ASNs under a single organisation, so you see the whole footprint.

Key takeaways

• An ASN uniquely identifies an autonomous system — a network routed under one policy.
• BGP uses ASNs to route traffic between networks, and every route carries the AS path of numbers it crossed.
• ASNs are 16-bit or 32-bit, allocated by the RIRs; only multihomed networks need their own.
• Networks relate as providers, peers, and customers — real topology that ipdex surfaces per ASN.
• An ASN is the bridge from a raw IP address to the real-world operator behind it, and to its place in the internet’s structure.