An autonomous system number identifies the network your traffic enters the internet from. It is coarser than your IP address, and harder to shake off.
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Which network carries your traffic onto the internet? Not the address it appears to use, but the organisation operating the connection — which a website can look up. Behind the address sits a larger identifier that describes the whole network, not an individual connection: the autonomous system number, or ASN. It is coarser than an address, and in an important way stickier.
You can see yours on the ASN lookup tool.
What makes an autonomous system
The internet is a federation of independently operated networks that agree to exchange traffic. Each is an autonomous system: a collection of addresses and routers under one administrative control, with its own routing policy.
A university, a large company, a mobile carrier and a cloud provider are each typically one or more autonomous systems. Your home broadband is not one — your provider is, and your address is a small piece of it.
Each autonomous system is identified by a number — the ASN. It is how networks announce to one another which addresses they can deliver traffic to, using a routing protocol called BGP.
Where the numbers come from
The numbers are allocated by regional internet registries, the bodies that hand out address space. Each allocation carries a public registration record: an organisation name, a country, contact details — what a whois lookup returns.
Routing data adds a second layer: because autonomous systems announce their blocks publicly, observers can see which ASN originates which addresses. Registry record plus observed routing gives a fairly reliable picture of who owns an address. The ASN lookup does that work for you, showing the number and the organisation behind it.
Reading the organisation name
The organisation name is the interesting part, because it tells you what kind of network is carrying you.
- A consumer access provider. Residential or mobile broadband — the ordinary case.
- A cloud or hosting provider. Traffic entering from a data centre, which is what a VPN, a proxy or a rented server looks like from outside.
- An enterprise or institution. A corporate network or a university, often with a fixed, recognisable range.
- A mobile carrier. Frequently a large autonomous system with a wide spread and a shifting pool of addresses.
That category is often more telling than the address. A residential address suggests an ordinary visitor; a hosting address suggests something less ordinary, and is one of the strongest hints that you are behind a tunnel, a subject we examine in can websites tell you use a VPN.
Coarser, but stickier
An IP address changes: the lease expires, you reconnect, the carrier rotates its pool. An ASN changes much less often.
Visit a site from three addresses in a month and, if all belong to the same autonomous system, the site can still group those visits as probably related. The ASN is coarser than an address, but more durable: not a fingerprint by itself, but a slow-changing attribute that pairs with the signals described in what is browser fingerprinting.
It has a second, subtler use. Because it reflects routing, it can reveal structure an address alone does not. Traffic that appears to come from a data centre while your timezone says otherwise is informative; the IP geolocation test places that estimate beside the ASN.
What changes it, and what does not
- A VPN replaces the ASN with the VPN provider’s. If that provider runs its own network the number is distinct; if it rents capacity from a larger host, the number may be shared with unrelated services, which weakens the association.
- A residential proxy also replaces it, with a consumer provider’s ASN, which is why it is harder to tell from an ordinary visitor than a data centre exit.
- Changing your IP within the same provider does not change it. A new address leaves the ASN identical, which is the point of calling it sticky.
- Tor changes it too, to a relay operator’s network, though many relay addresses are publicly listed and easy to spot.
The proxy and VPN detection test combines these signals into a single readout, faster than checking each by hand.
Where this leaves your address
Look up your own network. Run the ASN lookup and note the number and the organisation behind it. If the name is a consumer broadband provider, your connection looks ordinary. If it is a hosting company, you have just seen what a website sees.
That is only one layer of the picture. For a close reading of the same connection from the other end, what your IP address reveals takes the address apart and shows where the certainties stop.