Glossary

ASN (Autonomous System Number)

An ASN is a unique number identifying a network of IP ranges under one operator, used by BGP to route traffic across the internet.

An ASN (Autonomous System Number) is a globally unique number that identifies an autonomous system — a collection of IP address ranges (prefixes) managed under a single, clearly defined routing policy. Every large hosting provider, ISP, and cloud platform owns at least one ASN.

What it is for

ASNs are the unit that the Border Gateway Protocol (BGP) uses to exchange routing information. When you load a website, BGP decides the path between autonomous systems using their ASNs.

Assigned by Regional Internet Registries (RIRs) such as ARIN, RIPE, and APNIC.
Written as AS13335 (Cloudflare) or AS15169 (Google).
16-bit ASNs are largely exhausted, so 32-bit ASNs are now standard.

Why it matters for hosting

Looking up the ASN behind an IP tells you who actually operates the network — often the real datacenter or cloud provider, even when a site hides behind a CDN. It is the most reliable signal for attributing infrastructure ownership.

See also

IP Address An IP address is the unique numeric label assigned to each device on a network, used to route data to the right destination across the internet. CDN (Content Delivery Network) A CDN is a worldwide network of edge servers that cache and serve your site close to visitors, cutting latency and absorbing traffic spikes. Datacenter A datacenter is a secure facility housing servers, storage, and networking with redundant power and cooling, where your website is physically hosted. Reverse DNS (PTR) Reverse DNS resolves an IP address back to a hostname using a PTR record, commonly checked to verify mail servers and identify hosts.

← All terms