What is an ASN, and why does it matter?

If you've ever looked up an IP address on a tool like ours, you've probably seen a line that reads something like AS15169 GOOGLE or AS13335 CLOUDFLARENET. That number — the AS number, or ASN — is one of the most useful pieces of information you can get about an IP, and yet a lot of people glide past it without knowing what it means.

This article explains what an ASN is, why the internet needs them, and the practical reasons you'd want to look one up.

The internet is a network of networks

The "inter" in "internet" is doing a lot of work. The internet isn't a single network — it's tens of thousands of separate networks, run by different organizations, that have agreed to exchange traffic with each other. Each of these networks is called an autonomous system, or AS.

An AS is a network under a single administrative policy. Google is one AS. Cloudflare is one AS. Comcast is one AS (well, a few). Your university probably has one. A regional ISP in Lithuania might have one. They all interconnect at peering points and through transit providers, and together they form the internet.

Each AS gets a unique number — its ASN — so that the global routing system can refer to it. As of 2026, there are about 120,000 ASNs actively announcing routes on the global internet.

How ASNs are used in routing

The protocol that makes the global internet work is called BGP (Border Gateway Protocol). BGP is what each network uses to tell its neighbors "I can reach the following IP blocks." A BGP announcement looks roughly like this:

Prefix: 8.8.8.0/24
Path: AS15169

This says "I can deliver packets destined for 8.8.8.0/24, and the destination network is AS15169 (Google)." When this announcement propagates across the internet, every network that hears it adds its own ASN to the path, building up a chain like:

8.8.8.0/24 via AS3356 AS15169

That means: "to reach Google's block, go through AS3356 (Level 3 / Lumen) which connects to AS15169 (Google)." Routers everywhere use these path lists to decide which neighbor to forward a packet to.

This is why an ASN is a strong signal about who actually operates an IP: the ASN is the network that's announcing the block to the rest of the world. It's much harder to fake than a WHOIS record.

Reading an ASN line

An ASN line in a lookup result usually looks like one of these:

AS15169 Google LLC
AS13335 Cloudflare, Inc.
AS16509 AMAZON-02
AS7922 Comcast Cable Communications
AS3320 Deutsche Telekom AG

The number is the unique identifier. The text after it is the human-readable name registered with the regional registry. Most large companies have multiple ASNs — Amazon, for example, runs AS16509 (AWS), AS14618 (also AWS, in a different region), and several others. Google similarly runs different ASNs for different parts of its network.

Why this is useful

1. Knowing if a request is from a datacenter

If the ASN belongs to AWS, Google Cloud, Azure, Hetzner, OVH, DigitalOcean, or any other cloud provider, the IP is very likely a server — not a human's browser. This is one of the strongest signals for bot detection. A residential user on Comcast looks completely different from a request originating in AS16509.

2. Distinguishing ISPs from corporate networks

If you see AS3320 (Deutsche Telekom) on an incoming request, you're talking to a residential or small-business customer of DT. If you see AS-PRIVATE-CORPORATE-1234, you might be looking at a request coming through a specific company's outbound proxy. This is useful for compliance, audit logging, and understanding your traffic.

3. Spotting compromise patterns

Some ASNs are well known for serving as hosting for low-reputation operators — bulletproof hosters, abuse-tolerant networks, residential proxy aggregators. Security teams routinely keep watchlists of "high-abuse" ASNs to weight their fraud and intrusion detection systems.

4. Capacity planning and peering

If your CDN logs show that 30% of your traffic to Europe transits AS6939 (Hurricane Electric), you have data you can use to negotiate direct peering, reduce latency, and lower transit costs.

5. Geographic context that geolocation can miss

If the ASN is, say, AS37100 (SEACOM, an African carrier), the request is almost certainly from somewhere in their footprint — even if the city-level geolocation is uncertain. ASN gives you a "who", which often constrains the "where" better than a fuzzy IP-to-city lookup.

How ASNs are allocated

ASNs come from the regional internet registries, the same five organizations that allocate IP addresses: ARIN, RIPE, APNIC, LACNIC, AFRINIC. You apply, demonstrate that you need one (typically by showing you have your own address space and connections to multiple upstream providers), and pay a yearly maintenance fee.

For a long time, ASNs were 16-bit numbers — limited to about 65,000 unique values. The internet ran into this ceiling and switched to 32-bit ASNs, which give us a comfortable 4 billion. The transition was smoother than IPv4-to-IPv6, partly because ASNs are only relevant to the routing protocol, not to end users.

Looking up ASN data

Several services let you query ASN information directly:

WHOIS — the classic source. whois AS15169 from a Unix shell returns the registration record.
bgp.he.net — Hurricane Electric's BGP looking-glass; very rich, very searchable.
RIPEstat — RIPE's web interface to global routing data.
Our IP Lookup tool — shows the ASN that announces any IP you query.

The history, briefly

ASNs date back to the early days of multi-network routing. When the internet was still largely the ARPANET in the 1980s, the original routing protocol — EGP (Exterior Gateway Protocol) — required every network to have an identifier. ASNs were assigned starting at 1, and a handful of early numbers went to ARPA-funded sites, universities, and the early commercial backbones. AS1 was BBN, the contractor that ran the ARPANET. AS8 was the Massachusetts Institute of Technology. These low numbers are still in use today, which makes a few of them historical artifacts as much as operational identifiers.

When EGP gave way to BGP in 1989, the ASN system carried over. Over the next two decades, the rate of new ASN assignments climbed steadily as more and more organizations joined the internet at the routing level. The system has been stable for so long that BGP and ASN assignment are now considered some of the most boring, reliable infrastructure on the internet.

Public vs private ASNs

Just like IP addresses have ranges reserved for private use, ASNs have a private range: 64512–65534 for 16-bit ASNs, and 4200000000–4294967294 for 32-bit ASNs. These are used internally by some large networks for internal BGP without needing a globally unique number. You'll occasionally see them in traceroute output if a network shares them, but they should never appear in the global routing table.

A small worked example

Look up 1.1.1.1 in our lookup tool. You'll see something like:

ASN: AS13335 Cloudflare, Inc.
ISP: Cloudflare
Country: AU (or US, depending on routing)

The IP is Cloudflare's public DNS resolver. The ASN tells you definitively that the address is part of Cloudflare's network. The "country" might show as Australia (registry record) or the US (where the closest anycast PoP terminated your request). The ASN gives you the most reliable answer to "whose network is this?".

Try it: Look up some familiar IPs — your own, your favorite website's, a public DNS server — and compare the ASN field. You'll quickly start recognizing the same ASNs across services: Cloudflare, Google, Amazon, Microsoft, and a few major ISPs do a remarkable fraction of all internet traffic.