What is an ASN (Autonomous System Number)?
Discover what Autonomous System Numbers are, how BGP routes traffic between major networks, and how ASNs identify ISPs and tech giants.
The global Internet is not a single centrally managed computer network. Instead, it is an interconnected cooperative federation of over 100,000 distinct networks called Autonomous Systems (AS).
To coordinate traffic routing between these networks, each autonomous system is assigned a globally unique identifier called an Autonomous System Number (ASN).
Definition of an Autonomous System
An Autonomous System is a collection of connected IP routing prefixes controlled by one or more network operators that presents a single, clearly defined external routing policy to the rest of the Internet.
Examples of organizations operating their own Autonomous Systems include:
- Major telecommunications carriers (AT&T, Verizon, Deutsche Telekom, NTT)
- Global Cloud Providers and CDNs (Google AS15169, Cloudflare AS13335, Amazon AS16509, Fastly AS54113)
- Large financial institutions and academic universities
How BGP Utilizes ASNs
Autonomous Systems communicate with each other using the Border Gateway Protocol (BGP).
When you send a packet from a home connection in London to a web server in Singapore:
- Your local ISP’s network (e.g., AS2856 British Telecom) inspects the destination IP.
- The ISP checks its BGP routing table to see which adjacent AS provides the shortest AS path.
- The packet travels hop-by-hop across Autonomous Systems:
AS2856→AS1299(Arelion transit backbone) →AS15169(Google edge).
The ASN functions as the primary “zip code” for continental and international internet transit.
16-Bit vs 32-Bit ASNs
Originally, ASNs were 16-bit integers, providing 65,536 possible identifiers (numbers 1 to 65,535).
As the Internet expanded exponentially in the 2000s, this pool approached exhaustion. In 2007, RFC 4893 introduced 32-bit ASNs, expanding the allocation space to over 4.2 billion numbers ($2^{32}$).
Today, ASNs are categorized into:
- Public ASNs: Assigned by Regional Internet Registries (ARIN, RIPE, etc.) and routable across global BGP tables.
- Private ASNs: (e.g., 64512–65534 for 16-bit, and 4200000000–4294967294 for 32-bit). Used for private BGP peering inside data centers, cloud VPCs, or private corporate WANs.
How ASNs Help in IP Lookups
When looking up an IP address, knowing its ASN tells you:
- The Administrative Network Authority: You immediately know which organization is legally responsible for routing and network abuse enforcement for that subnet.
- Transit Quality: You can see whether traffic routes through Tier-1 global transit carriers or smaller local downstream providers.
- Datacenter vs Residential Identification: IPs belonging to ASNs like DigitalOcean (AS14061) or Hetzner (AS24940) represent cloud servers, while ASNs like Comcast (AS7922) represent residential home users.
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