ASN explained: what an Autonomous System Number actually is

When you look up your IP address, the result includes a line that reads something like AS7922 — Comcast Cable Communications or AS15169 — Google LLC. That AS-prefixed number is called an Autonomous System Number, and it's one of the most useful but least-explained pieces of data in a standard IP lookup.

Most internet users have never noticed it. Network engineers think about it constantly. Understanding what it means changes how you read any IP result — and it's simpler than it sounds.

What an Autonomous System actually is

An Autonomous System (AS) is a collection of IP networks operated by a single organisation under a single, coherent routing policy. The keyword is "autonomous" — it means the organisation manages its own routing decisions internally, and presents a unified face to the rest of the internet.

Comcast is one Autonomous System. A regional ISP in Germany is one. Your university almost certainly is one. Amazon Web Services runs several, organised by service and region. Google runs a handful. A content delivery network like Cloudflare is one, even though it operates data centres across hundreds of cities worldwide — from the outside, it has one routing identity.

The internet itself, at the routing layer, is just these systems talking to each other. When engineers say "the internet is made of about 100,000 Autonomous Systems", that's not a metaphor — it's a description of how the global routing table is actually structured. Every public IP address on the internet belongs to one of them.

Private addresses — the 192.168.x.x and 10.x.x.x ranges covered in the public vs private IP guide — don't belong to any AS, because they never appear on the public internet.

The number itself

An Autonomous System Number is just a unique identifier assigned to each AS. Historically these were 16-bit integers — meaning the available range topped out at 65,535. That ran out, as these things tend to do, and the standard was expanded to 32-bit in 2007, opening up around 4.3 billion possible values.

You'll see ASNs written in two ways. The old notation is a plain integer: 15169. The new "dotted" notation for 32-bit ASNs writes them as two 16-bit halves separated by a dot: 1.64512. In practice, most people and tools still write them as plain integers with the AS prefix — AS15169 — and it's almost always unambiguous.

ASNs are assigned by Regional Internet Registries — the same organisations that allocate IP address blocks. ARIN handles North America, RIPE NCC covers Europe and the Middle East, APNIC serves Asia-Pacific, LACNIC covers Latin America, and AFRINIC handles Africa. An organisation that wants its own AS applies to the relevant RIR, just as it would for an IP address allocation.

Currently around 100,000 ASNs are active globally. The number grows steadily as more organisations build out internet infrastructure, but it grows much more slowly than the IP address space — there are far fewer distinct networks than there are distinct IPs.

How ASNs and IP prefixes connect

Owning an AS doesn't mean much on its own. What an AS actually does is announce IP prefixes to the rest of the internet.

An IP prefix is a block of addresses — something like 8.8.8.0/24, which covers the 256 addresses from 8.8.8.0 through 8.8.8.255. (If the slash notation is unfamiliar, the CIDR calculator breaks down exactly what any prefix covers.) An organisation that operates an AS gets allocated one or more of these prefix blocks and then broadcasts to the internet: "to reach these addresses, route traffic through us".

That broadcast happens via a protocol called BGP — Border Gateway Protocol — the routing protocol that ties the global internet together. Every announcement carries the originating ASN as a label. Neighbouring networks receive it, record it, and pass it on. Within minutes, every major network on earth knows that 8.8.8.0/24 is reachable via AS15169 (Google).

This is how the mapping from IP address to ASN works in practice: an IP lookup tool queries a database of BGP announcements, finds which prefix the IP falls inside, and reads off the ASN that announced it. The geolocation data — city, country, coordinates — comes from a separate database with different accuracy characteristics. The ASN comes straight from the routing table, which makes it significantly more reliable.

How to find an ASN

The fastest way to look up the ASN for any IP is the IP lookup tool here on Network Lookup — it shows the ASN and the organisation name alongside the geolocation data for whatever address you enter.

Going the other direction — starting with an ASN and seeing everything it announces — is what the ASN explorer is for. Enter an ASN and you can see all the prefixes it currently announces, the organisation behind it, and related metadata.

Why ASNs are useful

There are three situations where knowing the ASN tells you something the IP address alone doesn't.

Geolocation context. As the IP geolocation post explains, city-level coordinates in an IP lookup are frequently wrong — sometimes by hundreds of miles. But the ASN is almost never wrong about the organisation. AS16509 is Amazon Web Services regardless of which data centre the IP is physically in. AS13335 is Cloudflare regardless of which country the edge node is located. The ASN tells you who's running the network; the geolocation data tells you approximately where — and the ASN is the more reliable of the two.

VPN and proxy detection. Most commercial VPN services and hosting providers run on a fairly small set of well-known Autonomous Systems. A residential internet connection will show an ASN belonging to a consumer ISP. A connection that shows AS14061 (DigitalOcean), AS16276 (OVH), or AS8100 (QuadraNet) is almost certainly coming from a cloud server or a VPN exit node rather than a home network. This is exactly how streaming services, fraud detection systems, and access-control tools distinguish between residential users and tunnelled connections — the ASN is a far more reliable signal than the geolocation coordinates.

Diagnosing network problems. When a website becomes unreachable from some parts of the internet but not others, the diagnosis usually involves looking at which ASNs can see the correct BGP announcements for the affected prefixes and which can't. A misconfigured route announcement, a transit provider outage, or a BGP hijack will show up as an inconsistency in which ASNs are propagating which routes. Network engineers track this in real time; the ASN layer is where the internet's routing problems become visible.

Tier 1, Tier 2, Tier 3

Not all Autonomous Systems are equal in the internet's hierarchy. There's an informal but widely used taxonomy based on how each AS pays for its connectivity.

A Tier 1 network is one that can reach every other network on the internet without paying anyone for transit. These are the backbone carriers — about a dozen networks worldwide, including names like Lumen (formerly CenturyLink), Telia, NTT, and Cogent. They reach each other through settlement-free peering agreements: "I'll carry your traffic if you carry mine". No money changes hands at the Tier 1 level because each network is roughly equal in value to the other.

A Tier 2 network buys transit from one or more Tier 1 networks but also peers freely with other networks of similar size. Most large regional ISPs and national carriers fall here. They pay to reach parts of the internet they don't directly connect to, but they also offset those costs by peering — exchanging traffic directly, for free, with other networks at internet exchange points.

A Tier 3 network buys all its transit upstream and does little or no free peering. Small local ISPs, corporate networks with their own AS, and many enterprise networks fall here. They pay for every bit of internet connectivity.

This taxonomy is informal — there's no standards body that assigns tiers — but it's the vocabulary network engineers use when talking about the economics of internet connectivity. As a consumer on a residential connection, you're sitting at the end of a chain that goes: your device → your ISP (Tier 2 or 3) → transit provider (Tier 1 or 2) → destination network. Each link in that chain corresponds to at least one AS handoff.

What the ASN in your IP lookup actually tells you

When you see AS7922 — Comcast Cable Communications in a lookup result, that single line tells you: this IP was allocated to Comcast, Comcast has announced it to the global routing table under their AS, and the address is presumably being used by one of Comcast's residential or business customers somewhere in the United States.

When you see AS15169 — Google LLC, you know you're looking at Google's infrastructure — could be a Google server, a Google Cloud customer's instance, or Google's DNS resolvers. When you see AS13335 — Cloudflare, Inc., you know the IP belongs to Cloudflare's network, which means the actual origin server is hidden behind it.

The city in the geolocation data might be wrong by a few hundred miles. The ASN is almost always right about the organisation. It's the line worth reading first.