“The internet” is not one network. It is tens of thousands of independently operated networks — internet providers, mobile carriers, cloud platforms, universities, content companies — that agree to exchange traffic. Each of those networks is an autonomous system (AS), and each has a unique autonomous system number (ASN).
What makes a network “autonomous”
An autonomous system is a network under a single administrative control with its own routing policy. Your ISP is one. Google is one (AS15169). Cloudflare is one (AS13335). A large company or university might be one if it connects to several providers.
ASNs are assigned by the regional internet registries. They were originally 16-bit numbers (up to 65,535); since RFC 6793 they are 32-bit, and many newer networks have numbers above 100,000.
BGP: how networks find each other
Autonomous systems exchange reachability information with the Border Gateway Protocol (BGP). Each AS announces the IP prefixes it originates — “8.8.8.0/24 is reachable through me” — and passes along announcements it learned from others, adding its own number to the AS path.
When your packet travels to a website, routers choose a path based on those announcements and on each network’s policy: which neighbours it prefers, which routes it accepts and what it pays for.
Upstreams, downstreams and peers
Networks connect in roughly three ways:
- Transit: a smaller network pays a larger one (its upstream) to reach the rest of the internet.
- Peering: two networks exchange traffic between their own customers directly, often for free, at internet exchange points or private interconnects.
- Customers of a network are its downstreams.
These business relationships aren’t published in BGP. They can only be inferred from the AS paths that route collectors observe — a network that appears just before another in many paths is probably its upstream.
Why ASNs are useful
- Troubleshooting: knowing which network an address belongs to tells you whom to contact, and whether a problem is local or upstream.
- Security: abuse reports, blocklists and DDoS mitigation often work per network.
- Understanding the internet: an ASN page shows every prefix a network announces in the global routing table, how much address space that covers, and when and where the number was registered.
Look one up
Every IP lookup shows the origin ASN of the address. Or explore networks directly: AS15169 (Google), AS13335 (Cloudflare), AS9121 (Türk Telekom).