What Is an ASN (Autonomous System Number)?
What is an ASN?
An autonomous system (AS) is a collection of IP address blocks operated under a single, coherent routing policy — in practice, one organization's corner of the internet. An autonomous system number (ASN) is the unique integer that identifies that network to every other network on Earth. Your internet provider has one. Google has several. So does your university, your cloud host, and roughly 120,000 other operators. When we trace an IP address, the ASN is often the single most informative field in the dossier — more so than the map pin.
The internet is a network of networks
The internet has no central operator. It is tens of thousands of independently run networks that agree to exchange traffic. Each of those networks needs a stable identity so the others know who is announcing which addresses, and that identity is the ASN. "AS15169" means the same thing to a router in Nairobi as it does to one in Oslo: Google's network. Everything inside an autonomous system is that operator's business; the ASN is the label on the outside of the box.
Who hands out ASNs
ASN allocation follows the same hierarchy as IP addresses. IANA (the Internet Assigned Numbers Authority) allocates blocks of numbers to the five Regional Internet Registries — ARIN for North America, RIPE NCC for Europe and the Middle East, APNIC for Asia-Pacific, LACNIC for Latin America, and AFRINIC for Africa. The RIRs then assign individual ASNs to network operators who can demonstrate a need, typically because they connect to more than one other network. ASNs were originally 16-bit values (0–65535); when those began to run short, RFC 6793 extended them to 32 bits, which is why you now see ASNs in the hundreds of thousands and beyond.
BGP, in one paragraph
Autonomous systems talk to each other using the Border Gateway Protocol (BGP). Each AS announces to its neighbours, in effect, "I can reach these address blocks," and passes along announcements it has heard from others. Every announcement carries the chain of ASNs it travelled through, so a router choosing where to send a packet can compare paths the way you might compare driving routes — usually preferring the one that crosses the fewest networks, adjusted by each operator's commercial and technical preferences. There is no central map and remarkably little verification: BGP is a web of neighbourly assertions, which is why a single mis-announcement can briefly redirect traffic for millions of people.
Famous ASNs
A few ASNs turn up constantly in server logs and traces:
| ASN | Operator | What it usually means in a trace |
|---|---|---|
| AS15169 | Google LLC | Google services and crawlers, including the public DNS resolvers at 8.8.8.8 |
| AS13335 | Cloudflare, Inc. | A proxy or CDN edge in front of the real origin server, not the origin itself |
| AS16509 | Amazon.com, Inc. | Something hosted on AWS — anything from a startup's API to a scraper |
Why the ASN often beats the coordinates
Geolocation coordinates are estimates with honest limits — country-level lookups are reliable, but city-level accuracy runs roughly 50–75% even on fixed broadband, as we detail in how accurate IP geolocation is. The ASN, by contrast, is a matter of public registry record, and it answers questions a map pin cannot:
- Connection class. A residential ISP's ASN, a mobile carrier's, and a cloud provider's imply very different things about the traffic. Uplink Trace flags datacenter and VPN ranges heuristically from the AS operator's name — a "LIKELY" rating, never proof.
- Abuse contact. Registries publish a responsible contact for each AS. If an address is attacking your server, the ASN tells you exactly whom to notify.
- Network reputation. Some ASNs are long-established carriers; others are bulletproof hosts that appear on every blocklist. The number carries history that coordinates never will.
None of this identifies a person. An ASN names a network operator, and IP geolocation as a whole cannot single out an individual or a household — a point we cover in whether someone can find your address from your IP.
Private-use ASNs
Just as some IP ranges are reserved for private networks, some ASNs are reserved for private use: 64512–65534 in the 16-bit space and 4200000000–4294967294 in the 32-bit space, per RFC 6996. They are meant for internal routing inside organizations and must never appear on the public internet. If one shows up in a trace or a routing table, it is almost certainly a configuration leak rather than a real network identity.
Look one up
To see what a number resolves to — operator name, registry, and the prefixes it announces — run it through our ASN lookup, or trace any address from the main tracer and read the AS field in the resulting dossier.