AS Numbers Index - Browse Autonomous Systems

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An AS number (ASN) uniquely identifies a network that controls a specific set of IP addresses and maintains its own routing policy. Use rtsak.com to explore the global registry of Autonomous Systems.

What is an Autonomous System?

An Autonomous System is a connected group of IP networks under the control of one or more network operators with a common routing policy. Every network that participates in BGP routing needs an AS number.

ASNs are assigned by Regional Internet Registries (RIRs):

  • ARIN - North America
  • RIPE NCC - Europe, Middle East, Central Asia
  • APNIC - Asia Pacific
  • LACNIC - Latin America and Caribbean
  • AFRINIC - Africa

AS Number Ranges

  • 1-65535 - 16-bit ASNs (original range, mostly allocated)
  • 65536-4294967295 - 32-bit ASNs (extended range, 4-byte ASNs)
  • 64512-65534 - Reserved for private use
  • 23456 - Reserved for AS_TRANS (4-byte ASN transition)

Major AS Numbers

Some of the largest autonomous systems by prefix count and traffic:

  • AS15169 - Google
  • AS13335 - Cloudflare
  • AS16509 - Amazon (AWS)
  • AS8075 - Microsoft
  • AS32934 - Facebook (Meta)
  • AS14618 - Amazon
  • AS20940 - Akamai

How to Use AS Lookup

Enter an AS number in the search box (e.g., AS15169 or just 15169) to view:

  • Organization details - Company name, country, and registration date
  • Announced prefixes - All IPv4 and IPv6 blocks advertised via BGP
  • Upstream providers - Transit relationships visible in BGP paths
  • Downstream customers - Networks receiving transit from this AS
  • BGP path analysis - Routing policy and path characteristics
  • IXP presence - Internet exchange points where the AS peers

Use Cases for AS Analysis

Threat intelligence - Identify which network hosts malicious infrastructure. Some ASNs have reputations for hosting abuse; knowing the ASN behind an IP provides immediate context.

Transit selection - Compare potential upstream providers by their prefix counts, geographic reach, and peering relationships.

Prefix hijack detection - Monitor whether your prefixes appear in BGP from unauthorized ASNs.

IP attribution - When investigating an IP address, the ASN reveals the responsible organization more reliably than WHOIS data, which may show resellers or intermediaries.

AS Macros

AS-SETs (also called AS macros) group multiple AS numbers for routing policy. They enable ISPs to express complex routing relationships in a maintainable way.

→ Browse AS Macros

FAQ

How do I get an AS number?
Request one from your Regional Internet Registry (ARIN, RIPE, APNIC, LACNIC, or AFRINIC). You need to demonstrate technical need and typically need to be multihomed or plan to peer at an IXP.
What's the difference between 16-bit and 32-bit ASNs?
16-bit ASNs (1-65535) were the original format. As these became scarce, 32-bit ASNs (up to 4.3 billion) were introduced in 2007. Both function identically in modern BGP.
Can I see which ASN an IP address belongs to?
Yes, look up the IP address to see its origin AS - the network announcing that prefix via BGP.
What is an AS-SET?
An AS-SET (or AS macro) is a named group of AS numbers used in routing policy. It lets ISPs define customer cones and peering relationships without listing every individual ASN.
How do I find my organization's ASN?
Check your IP allocation from your regional internet registry, or query any IP address you control to see its announcing AS.
What's the difference between transit and peering?
Transit is paid connectivity where one AS carries another's traffic to the full internet. Peering is mutual exchange of customer traffic, typically settlement-free.
Why do some ASNs show no prefixes?
The AS may exist but not currently announce routes (dormant), or may be used only for internal purposes without public routing.
Can one organization have multiple ASNs?
Yes. Large organizations often operate several ASNs for different business units, geographic regions, or technical purposes.
What's the difference between ASN and AS?
They're used interchangeably. AS (Autonomous System) is the network itself; ASN (Autonomous System Number) is its identifier.
  • Prefixes - IP prefix & BGP route analysis