What is an ASN?
An Autonomous System Number (ASN) is a unique identifier allocated to autonomous systems (AS) for use in BGP (Border Gateway Protocol) routing. An autonomous system is a collection of IP networks under the control of a single organization that presents a common routing policy to the internet.
ASNs are essential for internet routing. They allow different networks to exchange routing information and determine the best paths for data to travel across the internet. Every major ISP, hosting provider, and large organization that manages its own network has at least one ASN.
How ASNs Work
The internet is composed of thousands of autonomous systems, each identified by its ASN. These systems use BGP to:
- Announce Routes: Tell other networks which IP addresses they control
- Exchange Information: Share routing tables with neighboring networks
- Find Paths: Determine the best route for data packets
- Handle Failures: Reroute traffic when links go down
ASN Format and Allocation
ASNs are allocated by Regional Internet Registries (RIRs):
- 16-bit ASNs: Range from 1 to 65,534 (mostly exhausted)
- 32-bit ASNs: Range from 65,536 to 4,294,967,295
- Reserved ASNs: 64,512-65,534 for private use
Why ASNs Matter
- Network Identity: Identifies the network operator for an IP address
- Routing Decisions: Determines how traffic flows across the internet
- Security: Helps identify and block malicious networks
- Peering: Enables direct connections between networks
- Troubleshooting: Assists in diagnosing routing problems
Looking Up ASN Information
ASN information can reveal:
- The organization that owns the IP address block
- The size of the network (number of IP addresses)
- Peering relationships with other networks
- Geographic presence of the network
- Upstream providers and connectivity