Glossary · Networking

ASN (Autonomous System Number)

An ASN is a unique number assigned to a network operator that identifies its routing domain in BGP on the public Internet.

Updated October 6, 2026

An ASN (Autonomous System Number) uniquely identifies an autonomous system, a network or group of networks under one administrative control and routing policy. It is written like AS15169.

Why it matters

ASNs tell you who operates an IP address. Looking up the ASN for an address shows whether traffic goes to a cloud provider, ISP or CDN, which helps with abuse reports, blocklists and tracing outages. Regional Internet registries allocate ASNs, and the number appears in BGP paths as the networks pass routes along.

Example

$ whois -h whois.cymru.com " -v 8.8.8.8"
AS      | IP      | BGP Prefix | AS Name
15169   | 8.8.8.8 | 8.8.8.0/24 | GOOGLE

The address 8.8.8.8 is announced by AS15169, operated by Google.

Common mistakes

  • Assuming one company has only one ASN. Large operators often hold several.
  • Blocking an entire ASN without checking the legitimate services hosted on it.
  • Confusing an ASN with an IP range. It identifies the operator, which may announce many prefixes.

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