ASN
Also known as: Autonomous System Number
A globally unique number identifying a network of IP prefixes under a single administrative routing policy, used by BGP to route traffic across the internet.
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What is an ASN?
An ASN (Autonomous System Number) is a globally unique identifier assigned to an autonomous system (AS) — a network, or group of networks, operated under a single administrative routing policy. Every ISP, major cloud provider, CDN, and large enterprise that exchanges traffic directly with other networks has at least one ASN. The internet, at its highest level, is a graph of autonomous systems passing packets between each other using the Border Gateway Protocol (BGP).
How ASNs are assigned
ASNs are allocated by the Regional Internet Registries (RIRs):
- ARIN — North America
- RIPE NCC — Europe, Middle East, parts of Central Asia
- APNIC — Asia-Pacific
- LACNIC — Latin America and Caribbean
- AFRINIC — Africa
Originally ASNs were 16-bit (0 to 65,535). The space was extended to 32-bit in 2007 (RFC 6793), adding roughly 4 billion more ASNs. Numbers above 65,535 are 32-bit ASNs; older networks may still only support the 16-bit range.
What an ASN tells you about an IP
Looking up the ASN for any IP address reveals:
- Who owns the network — the organization name and country
- What other IP prefixes they own — a single AS often announces dozens or hundreds of CIDR blocks
- Peering relationships — which other networks it exchanges traffic with
This is useful for fraud detection (identifying VPN, datacenter, or hosting ASNs that residential users rarely come from), network troubleshooting (tracing where traffic is routed), and threat intelligence (correlating abuse reports across an entire AS).
Look up an ASN with the RDAP protocol or our IP lookup tool, which includes ASN data for every result.