guest@ipterm:~$ man asn
What is an ASN (autonomous system number)?
The internet is a network of networks. Each one that runs its own routing, such as an internet provider, cloud platform, university or large company, is an autonomous system, identified by an autonomous system number (ASN) like AS15169.
How ASNs route traffic
Autonomous systems exchange routes using BGP (Border Gateway Protocol). Each network announces the blocks of IP addresses, called prefixes, that it can deliver traffic to, and its neighbors pass those announcements along. When you load a page, your traffic crosses a chain of these networks to reach the one that announced the destination address.
Who assigns them
IANA hands out blocks of AS numbers to five Regional Internet Registries, which assign them to organizations:
- ARIN: North America
- RIPE NCC: Europe, the Middle East and parts of Central Asia
- APNIC: Asia-Pacific
- LACNIC: Latin America and the Caribbean
- AFRINIC: Africa
The numbers themselves
ASNs were originally 16-bit (1 to 65,535). 32-bit ASNs, up to 4,294,967,295, were added once those began to run short. Some ranges are reserved: 64512–65534 and 4200000000–4294967294 for private use (RFC 6996), and 64496–64511 and 65536–65551 for documentation (RFC 5398).
Why look one up
- Find out who operates an address, for example before reporting abuse or spam.
- Tell residential from datacenter traffic: a visitor from a cloud provider's network is more likely to be automated.
- Troubleshoot routing and reachability problems.
Every IP result in ipterm links to its network. Network pages show the holder, the registry, how many prefixes it announces and how much IPv4 address space they cover, using data from RIPEstat.
Try itLook up AS13335 (Cloudflare) or AS15169 (Google).