AS51557

An ASN (Autonomous System Number) identifies a single network — a collection of IP address ranges operated under one administrative authority, such as a cloud provider, CDN, hosting company, or ISP. Every ASN is a globally unique number (like AS13335 for Cloudflare or AS16509 for Amazon AWS).

Networks announce the IP ranges they own to the rest of the internet using BGP (Border Gateway Protocol). Those announcements are what let routers worldwide agree on a path to any address — so the ASN is effectively the answer to “whose network does this IP live on, and how do packets get there?” One operator often controls several ASNs, which is why we roll sibling networks together.

Because of that, the ASN is where DNS meets the physical internet:

• A domain’s A / AAAA records resolve to IP addresses, and every IP belongs to exactly one ASN — so the ASN reveals who actually serves a site, even when the DNS records don’t name the provider.
• It separates the network (e.g. Fastly) from the platform running on top of it (e.g. GitHub Pages), which a domain’s records alone can obscure.
• When a site’s IPs move from one ASN to another, that’s a real hosting or CDN change — the signal behind the joins and exits tracked on this page.