Glossary
Autonomous system number
An autonomous system is a network with its own routing policy, and an autonomous system number is the identifier it uses to announce which IP address blocks it carries. Looking up the ASN behind a website's address tells you which organisation actually operates the network the site is served from — a hosting provider, a CDN, a cloud region, or the company itself. It describes the network, not the server and not the site.
What an ASN identifies
A network operator with independent routing policy: an internet service provider, a hosting company, a cloud platform, a large enterprise, or a content delivery network.
Numbers are assigned by the regional internet registries, and the mapping from an address to an ASN is derived from the routing announcements those networks make.
One organisation may hold many ASNs, often by acquisition or by region, so a single name can appear under several numbers.
The number is stable over long periods, which makes it a more durable identifier of an operator than an address block that may be reassigned.
What it tells you about a site
Who actually serves the traffic. A site whose address belongs to a CDN's ASN is behind that CDN, whatever the application is written in.
Whether the origin is exposed. When an address resolves into a hosting provider's ASN rather than the CDN's, the origin server is reachable directly and the CDN can be bypassed.
Geographic and operational concentration: several services that appear independent resolving into one ASN share a failure domain.
Whether a claimed crawler is plausible. Combined with reverse DNS, the ASN of a connecting address is part of establishing that a request claiming to be from a search engine is.
What it does not tell you
Anything about the site's content, quality or security. An ASN is a routing fact.
Who owns the domain. The network operator and the domain registrant are usually different organisations entirely.
The physical location of the server. Address geolocation is inferred and frequently wrong, particularly on anycast networks where one address is announced from dozens of places.
Whether the hosting is shared or dedicated, which is invisible from outside.
How to look one up
Resolve the hostname to an address, then query a routing information service for the ASN announcing that address. Public registries and looking-glass services both expose this.
Check every address a hostname resolves to, not just the first. A dual-stack host may route IPv4 and IPv6 through different networks.
Compare the ASN of the public hostname against that of any origin hostname you know. Two different networks mean the origin is separately reachable.
Treat geolocation attached to the result as approximate. The ASN is reliable; the city is not.
Frequently asked questions
- What does an ASN tell me about a website?
- Which network operator serves it — a hosting provider, a cloud platform or a CDN. It says nothing about the content or the software.
- Can I find a server's location from its ASN?
- Only approximately. Geolocation of addresses is inferred, and anycast networks announce one address from many locations at once.
- Why does my site show a CDN's ASN?
- Because the CDN terminates the connection. That is the expected result when a CDN is in front of the origin.
Sources
Related
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