What Is an ASN? Autonomous System Numbers Explained
An ASN (Autonomous System Number) is a unique identifier assigned to a network or collection of networks that operate under a single administrative routing policy. In BGP (the protocol that routes traffic between networks), each autonomous system is identified by its ASN. Major ISPs, cloud providers, universities, and large organizations each have their own ASN. When you look up your IP address, the ASN tells you which network owns the IP block your address belongs to.
How ASNs Work
The internet isn’t one network. It’s a network of networks, roughly 75,000 actively interconnected autonomous systems. Each AS controls a specific set of IP address blocks and announces routes to those blocks via BGP.
For example:
- AS13335: Cloudflare
- AS15169: Google
- AS32934: Facebook/Meta
- AS16509: Amazon (AWS)
- AS8075: Microsoft
When your traffic travels across the internet, it passes through multiple autonomous systems. The path might look like: Your ISP (AS1234) → Transit provider (AS5678) → Destination’s AS (AS13335). BGP determines this path based on routing announcements and policies.
ASN Types
2-byte ASNs: The original format, numbers 0 to 65,535. These were running out, so…
4-byte ASNs: Extended format, numbers up to 4,294,967,296. Written in “asdot” notation (e.g., AS1.5) or “asplain” notation (e.g., AS65536). Required since the 2-byte space was nearly exhausted.
Public ASNs: Visible on the global internet, used in BGP routing between organizations.
Private ASNs: (64512 to 65534 for 2-byte, similar ranges for 4-byte) Used internally and not visible on the global internet. Useful for organizations with complex internal routing that don’t need globally unique ASNs.
Getting an ASN
To obtain an ASN, you need:
- A valid reason (multi-homed network, unique routing policy, becoming a transit provider)
- At least one upstream provider willing to peer with you
- Application to your regional internet registry (ARIN, RIPE, APNIC, etc.)
- An annual fee ($550 from ARIN, varies by registry)
Most small and medium organizations don’t need their own ASN. Your ISP handles routing for you. ASNs become necessary when you have multiple ISP connections (multi-homing) and need to control routing policy independently.
Test It Yourself
ASN Lookup
Enter any IP address to see its ASN, organization, and network details.