What is an Autonomous System (ASN)?
An Autonomous System Number (ASN) is a unique identifier assigned to a network or group of networks that share a single routing policy on the internet. ISPs, cloud providers, and large organizations each hold their own ASN to advertise which IP ranges they control via BGP.
Key ASN concepts
Download CSV| Term | Description | Why it matters | Example |
|---|---|---|---|
| ASN | The unique ID for an autonomous system | How it's identified for routing | AS13335 |
| IP Prefixes | The IP ranges that an ASN announces | Shows who owns which traffic | 104.16.0.0/12 |
| BGP | The protocol that handles routing between ASNs | Decides how traffic gets routed | AS_PATH |
| Upstream | Your provider's ASN | Gets you connected to the internet | Tier-1 ISP |
| Downstream | Your customer's ASN | Where traffic originally comes from | Hosting provider |
| Peering | Direct connection between two ASNs | Traffic goes straight between them | IXP peering |
| RIR | The organization that gives out ASNs | Controls who gets what | ARIN / RIPE |
| AS-SET | A collection of multiple ASNs | Used for setting routing policies | IRR objects |
How ASNs are used
- Moving traffic around the internet
- Figuring out who owns which IPs and who's responsible
- Setting up security rules and routing policies
ASN vs IP address vs ISP
- An IP address is what identifies a specific device.
- An ASN identifies the network that's announcing those IP ranges.
- An ISP is the actual company running things-they might operate one or more ASNs.