What is an ASN (Autonomous System Number)?
An ASN — Autonomous System Number — identifies a single network on the internet, usually a specific ISP, hosting company, or large organization, and it is the number the internet’s routers actually use to find your traffic a path home. It sits one layer below your IP address: your IP tells a site where to send a reply, and the ASN tells the internet’s backbone which network to hand that reply to next. It’s one of several things a bare address reveals about your connection, alongside your approximate location and ISP name — see our overview of what your IP reveals for the fuller picture.
Autonomous systems and BGP, in plain words
An “autonomous system” is a block of IP address space plus the routers that manage it, all under one organization’s control — a phone carrier, a university, a hosting provider, a whole country’s incumbent telecom. Each one gets a public AS number, and outside the network nobody needs to know its internal layout; the AS just announces “these address blocks are reachable through me” to its neighbors. Those announcements ripple across the internet through Border Gateway Protocol (BGP), the routing protocol that stitches tens of thousands of independently run networks into the single internet we use. When you visit a site, BGP is what decided, hop by hop, which autonomous systems your packet crossed to get there. Larger organizations sometimes run more than one autonomous system, or peer directly with several other networks rather than relying on a single upstream, which is part of why big ISPs and cloud providers show up across many different paths in a traceroute.
AS numbers with real examples
AS numbers look like AS15169 or AS7922. Some familiar ones: AS15169 is
Google, AS16509 is Amazon, AS3320 is Deutsche Telekom, AS7922 is
Comcast. Every large ISP, cloud provider, and CDN has one — usually several,
for different regions or business units. Small businesses and individuals
never get their own; they route through their provider’s AS instead. The
original numbering scheme was 16-bit, topping out at 65,535, and it filled
up as the internet grew, so a newer 32-bit range was opened to keep issuing
fresh ones; a separate block is also reserved for private use inside an
organization’s own network, the AS equivalent of the private IP ranges used
inside a home router.
How your ASN maps to your real ISP — even behind resellers
Your IP’s ASN almost always identifies your true underlying provider, even if the company you pay is a reseller. Many regional ISPs and mobile virtual network operators (MVNOs) lease network capacity from a larger carrier rather than building their own; your bill shows the reseller’s brand, but your traffic still exits through the parent network’s AS, and that is what shows up when a site looks you up. In the United States, a discount mobile carrier riding on a larger network’s towers is a textbook case; in much of Latin America, smaller regional ISPs reselling capacity from a national fiber operator work the same way. This is exactly what our ASN card on the home page shows: the organization actually carrying your connection, which can be more specific and more useful than the ISP name printed on your invoice.
Datacenter ASNs vs ISP ASNs
Sites can tell the two apart at a glance, because the AS itself usually says which kind it is. Residential and mobile ASNs belong to organizations in the business of connecting homes and phones — Comcast, Vodafone, América Móvil. Datacenter ASNs belong to hosting and cloud companies — DigitalOcean, OVH, Hetzner, AWS — whose customers are servers, not living rooms. Seeing a datacenter ASN attached to what should be a home visitor is one of the clearest signals that traffic is passing through a VPN, proxy, or hosted server rather than a residential line, which is exactly the check our VPN test runs against your own connection. This is also the core signal behind most commercial “IP intelligence” databases, which classify millions of address blocks by the ASN they belong to rather than inspecting traffic itself — fast, and accurate as long as the underlying ASN classification is kept up to date.