ASN Lookup & Autonomous System Directory
Explore global Autonomous System Numbers, inspect 16-bit and 32-bit BGP representations, verify RFC private blocks, and inspect Tier 1 transit backbones.
Cloudflare — Cloudflare, Inc. (US)
whois -h whois.radb.net AS13335Curated Global Autonomous Systems Directory (1)
Click any row to inspect| ASN | Entity | Type | RIR | IPv4 / IPv6 Prefixes | Inspect |
|---|---|---|---|---|---|
| AS13335 | CloudflareCloudflare, Inc. (US) | CDN / Edge | ARIN | v4: 2,800+ | v6: 1,400+ |
Border Gateway Protocol (BGP) & Autonomous System Numbers Explained
01.What is an Autonomous System (AS)?
An Autonomous System (AS) is a connected group of one or more IP prefixes run by one or more network operators that presents a single, clearly defined interior routing policy to the Internet.
To exchange routing paths with other networks across the global Internet, each autonomous network is assigned a globally unique Autonomous System Number (ASN) by one of the five Regional Internet Registries (RIRs: ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC).
Routers utilize the Border Gateway Protocol version 4 (BGP-4) to advertise which IP prefixes can be reached through their respective ASN paths.
02.16-bit vs 32-bit ASNs: ASPLAIN and ASDOT
Originally, ASNs were 16-bit integers (0 to 65,535). As the Internet grew, available 16-bit numbers dwindled, prompting the IETF to introduce 32-bit ASNs (RFC 6793) expanding the pool to over 4.29 billion addresses.
Two representation standards emerged (RFC 5396):
- ASPLAIN: Simple decimal integer representation (e.g.
65546). This is the universally adopted operational standard today across Cisco, Juniper, and Linux BGP daemons (FRR/BIRD). - ASDOT: Breaks the 32-bit integer into two 16-bit numbers separated by a dot:
high.lowwherehigh = Math.floor(ASN / 65536)andlow = ASN % 65536. (e.g.,1.10).
03.RFC 6996 Private ASN Ranges
Just as RFC 1918 reserves IP addresses for private LANs, RFC 6996 reserves ASN ranges for private enterprise routing (such as AWS Direct Connect, Azure ExpressRoute, or internal data center CLOS topologies):
- 16-bit Private Range:
64,512 – 65,534(1,023 available ASNs). - 32-bit Private Range:
4,200,000,000 – 4,294,967,294(94,967,295 available ASNs).
Upstream transit providers strictly filter and strip these numbers from external BGP route advertisements.
04.What Defines a Tier 1 Internet Transit Network?
A Tier 1 network (e.g., Lumen AS3356, Arelion AS1299, NTT AS2914, Cogent AS174) is an Internet Protocol network that can reach every other network on the global Internet solely via settlement-free peering.
Unlike Tier 2 or Tier 3 ISPs, a true Tier 1 operator pays nothing for IP transit; they maintain massive intercontinental optical backbones and trade symmetric bandwidth with fellow Tier 1 peers at major Internet Exchange Points (IXPs).