What is an ASN? Autonomous systems, explained
Sooner or later every log, report and routing question leads to a number written like AS15169, and everything else about the network hangs off it. An ASN is the closest thing the internet has to a company registration number for networks: one identity per operator that routes its own traffic. Here is what an autonomous system actually is, who gets a number, how to read one, and where the number stops telling you things.
The routing unit, not the company
An autonomous system is a set of networks under one routing policy: one operator deciding what address space it announces to the world and which neighbours it exchanges traffic with. The AS number, the ASN, is the identifier that policy operates under. It is deliberately not the same thing as a company. One company can run several autonomous systems for different regions or products, and one AS can carry space belonging to many customers. The AS is the unit the routing system sees, and when you research a network, it is the unit you actually investigate.
What the number does in BGP
BGP, the protocol that stitches independent networks into one internet, is a conversation between autonomous systems. Every route announcement ends in the origin ASN, the network claiming "this prefix is reached through me", and records the path of AS numbers the announcement travelled through. That is what the number is for: it makes networks accountable participants in routing rather than anonymous ones. It is also why so much of a block's story is written in ASN terms: which AS originated it, when the origin changed and whether that change was authorised, and which ASes carried the announcement.
Who gets an ASN, and how
The same five regional registries that allocate IP addresses issue AS numbers, and the classic justification is multihoming: a network connecting to more than one provider with its own routing policy needs its own identity to speak BGP under. Hosting companies, ISPs, content platforms, universities and plenty of ordinary enterprises hold one; a business that simply buys internet access from one provider does not need one and routes under its provider's instead.
Originally AS numbers were 16-bit, giving about 65,000 of them, and that pool got tight enough that 32-bit numbers were introduced in the late 2000s, extending the range into the billions. Both kinds work the same way; a low number is often just an old network. Like address space, some ranges are set aside: blocks of numbers reserved for private internal use and for documentation, which never appear in the public routing table and are the reason worked examples in careful writing use ASNs like AS64496 rather than a real network's identity.
Reading the number itself
The notation is just the number with an AS prefix: AS15169 is Google, AS13335 is Cloudflare, and the bare number means the same thing. The number itself carries almost no information; it is an opaque key, assigned in issue order, and reading anything into its magnitude beyond rough age is astrology. Everything interesting is what the key unlocks: the registration behind it, the space announced under it, and the history both have left.
What an ASN tells you
Treated as a research key, the number opens four doors, and together they profile a network better than any single record can.
- Identity. The registry record: who holds the number, in which country, since when, with which contacts. Self-reported, like all registration data, and still the starting point.
- Space. What the AS announces today, IPv4 and IPv6, and what that space is used for, which is where a network's character actually shows.
- Relationships. Its upstreams and customers, read from the routing table. The shape, a stub, a regional ISP, a transit backbone, is visible at a glance.
- History. What it has announced over time, what it stopped announcing, and what its space has been flagged as, dated. Networks change character, and only the history shows the change.
And what it does not
An ASN identifies the operator responsible for routing, not the person behind traffic. An address inside a big access network tells you the subscriber's ISP, not the subscriber, and behind carrier NAT one address is thousands of people. Nor does an ASN certify anything: getting a number is paperwork, and a tidy new ASN with a plausible name is exactly what space that wants a clean identity acquires first. The number is where questions start, not where they end, which is why the useful habit is never "what is this ASN" alone but "what has this ASN been": looked up with its registration, its announced space and its dated history side by side, the number becomes the most informative single key the internet's public record has.
Enter any AS number on the front page, with or without the AS prefix, to see its registration, announced space, relationships and history in one report. Every fact names its source and date, and the report says plainly when nobody has checked something.