What is an ASN? Autonomous System Numbers explained

Look up any IP address — say, 8.8.8.8 — and you'll see something labeled ASN AS15169. That's an Autonomous System Number. It's one of the most important identifiers in internet infrastructure that almost no normal user has heard of.

If you've ever wondered "who actually runs the internet," ASNs are part of the answer.

The two-sentence version

The internet is a network of networks. Each separately-administered network is an Autonomous System (AS), and it has a unique number — its ASN.

Think of ASNs as area codes for the internet. They identify which organization is responsible for a given chunk of address space and how their network connects to the rest.

How the internet is organized

You probably picture the internet as one big mesh, but operationally, it's tens of thousands of independently-run networks that have agreed to exchange traffic with each other. Examples:

AS15169 — Google. Owns the IP range that includes 8.8.8.8.
AS13335 — Cloudflare. Owns 1.1.1.1.
AS3356 — Lumen (formerly Level 3). A massive backbone provider.
AS7018 — AT&T Internet. Carries traffic for tens of millions of US subscribers.
AS55836 — Reliance Jio. India's largest mobile carrier.

When a packet leaves your device for an unfamiliar destination, it bounces between ASes until it reaches the AS that owns the destination IP. Each AS independently decides how it forwards traffic — which connections to use, which prices to pay, whose traffic to drop.

The protocol that makes this work is BGP (Border Gateway Protocol). BGP routers at the edge of each AS announce "I can reach these IP ranges" to neighboring ASes, and routers piece together the full map of who can reach what.

ASN format

ASNs come in two flavors:

16-bit ASNs: numbers from 1 to 65,535 (about 65,000 possible). The original allocation. Numbers below 64,495 are public; the rest are reserved for private use.
32-bit ASNs: numbers up to 4,294,967,295. Created in 2007 because the 16-bit pool was running out (similar problem to IPv4 → IPv6).

By convention, ASNs are written as AS followed by the number — AS15169, AS13335. Some tools omit the AS prefix; it's still the same identifier.

About 100,000 ASNs are currently in active use globally. Allocation is handled by the same five regional internet registries (RIRs) that allocate IPs:

ARIN (North America)
RIPE NCC (Europe, Middle East, Central Asia)
APNIC (Asia Pacific)
LACNIC (Latin America)
AFRINIC (Africa)

Who needs an ASN?

You need an ASN if you want to:

Connect to multiple ISPs with redundant routing (called multi-homing).
Operate your own routing policy — choose how your traffic exits, prefer some peers over others.
Run a public-facing infrastructure business — CDN, transit, large hosting.

Most companies, including most websites, do not have their own ASN. They rent space inside someone else's. A small business hosting a website at AWS uses AWS's ASN (AS16509). A company on Cloudflare uses Cloudflare's (AS13335). Only when you're running enough infrastructure to justify your own multi-homed network does getting an ASN make sense.

That said, getting one isn't terribly expensive — a few hundred dollars one-time plus annual RIR membership fees. The bigger barrier is the technical capability and IP space (you'd typically need a /24 or larger block to announce).

What ASN data tells you

When you look up an IP and see its ASN, you learn:

Who runs the network, with name and country.
How big the network is (number of allocated IP ranges).
What kind of network it is — residential ISP, business ISP, mobile carrier, hosting provider, content delivery network, enterprise.
Where they peer, if you dig into BGP databases.

This is genuinely useful for:

Detecting hosting / cloud traffic — bot traffic often comes from cloud ASNs (AWS, GCP, OVH, DigitalOcean). Residential ASNs are usually real users.
Geolocation context — mobile carrier ASNs hint that geolocation may be inaccurate (mobile IPs anchor to central network points, not subscriber locations).
VPN / proxy detection — VPN providers run their own ASNs. Comparing the apparent IP's ASN against a list of known VPN ASNs catches most masked traffic.
Network troubleshooting — if your traceroute jumps between unexpected ASNs, you're seeing routing problems.

How to look up an ASN

Several free options:

Our IP Lookup tool returns the ASN with every IP query.
Our WHOIS tool returns more detailed registration info.
bgp.he.net (Hurricane Electric) — the gold standard for deep BGP and ASN data. Search by ASN or IP and see peering relationships, announced prefixes, IPv6 reach, and history.
bgp.tools — newer, faster UI, similar data depth.
peeringdb.com — an opt-in directory where networks list their peering policies and exchange points.

ASN trivia worth knowing

AS0 is reserved — it explicitly means "no AS" and shouldn't appear in routing.
Private ASNs (64512–65534 in 16-bit, 4200000000–4294967294 in 32-bit) are like RFC 1918 private IPs — anyone can use them inside their own networks.
Some ASNs become famous. AS7007 caused the 1997 internet outage by accidentally announcing routes for the entire internet. AS4761 (Indosat) caused a 2014 outage by leaking routes from Google. Routing accidents happen and they hurt.
ASN 0:0 is the bogon ASN sometimes seen in test data.

TL;DR

An ASN identifies an independently-administered network on the internet. There are about 100,000 of them in active use. You'll see ASNs whenever you look up an IP — they tell you which organization runs the network the IP belongs to, which is useful for understanding context, debugging routing, and detecting bots or VPN traffic.

For a normal user: knowing your IP's ASN is mostly a curiosity. For a network engineer, it's the foundation of how the internet actually works.