What is an ASN?

// Introduction

An Autonomous System Number (ASN) is a unique identifier assigned to a collection of IP networks and routers under the control of a single organisation that presents a common routing policy to the internet. ASNs are the fundamental building blocks of internet routing.

Every time you send data across the internet, it passes through multiple autonomous systems — each one making routing decisions based on BGP (Border Gateway Protocol) announcements. Understanding ASNs helps explain why internet traffic sometimes takes unexpected paths, why certain regions experience outages, and how ISPs and cloud providers interconnect.

There are currently over 100,000 active ASNs worldwide, operated by ISPs, universities, cloud providers, enterprises, governments, and content delivery networks.

// ASN Format

ASNs were originally 16-bit numbers (range 1–65535), but the internet outgrew this. RFC 4893 (2007) extended ASNs to 32 bits, providing over 4 billion possible values.

ASNs are written as a plain number (e.g. AS15169) or in the older dot-notation for 32-bit values (e.g. 1.64512).

// What is an Autonomous System?

An Autonomous System (AS) is a network or group of networks that shares a single routing policy and is controlled by one administrative entity. The key word is autonomous — the organisation independently decides how traffic enters and exits its network.

An AS typically includes:

• One or more IP address prefixes (blocks of IP addresses)
• Border routers that connect to other autonomous systems
• Internal routers that route traffic within the AS
• A BGP routing policy that defines which prefixes to advertise and accept

Types of Autonomous Systems

• Stub AS — Has only one upstream provider. Traffic enters and exits through that single provider. Most small businesses and universities.
• Multihomed AS — Connected to multiple upstream providers for redundancy. No transit traffic passes through it.
• Transit AS — Carries traffic between other autonomous systems. ISPs and backbone networks are transit ASes.

// BGP — How ASNs Communicate

Border Gateway Protocol (BGP) is the routing protocol that autonomous systems use to exchange reachability information. BGP is often called the "protocol that runs the internet" — it is responsible for keeping track of all the paths between every network on earth.

When an AS wants to make its IP prefixes reachable, it announces them to its BGP peers. The announcement says, in effect: "To reach these IP addresses, send traffic to me." Neighbouring ASes propagate these announcements to their peers, and so on, until the prefix is known globally.

BGP Path Selection

When multiple paths to a destination exist, BGP selects the best path using a priority-ordered set of attributes:

1. Highest Local Preference (internal policy)
2. Shortest AS-PATH (fewest hops between ASes)
3. Lowest MED (Multi-Exit Discriminator)
4. eBGP over iBGP routes
5. Lowest router ID as a tiebreaker

// Who Assigns ASNs?

ASNs are assigned by the five Regional Internet Registries (RIRs) — the same organisations that manage IP address allocation:

To obtain an ASN, an organisation must have a legitimate need for a unique routing policy (typically multi-homed connectivity) and pay an annual membership fee to their RIR. The process involves technical verification and takes days to weeks.

// IP Prefixes and Route Announcements

Each AS announces one or more IP prefixes — blocks of IP addresses expressed in CIDR notation. For example, Google's AS15169 announces hundreds of prefixes including:

The global BGP routing table (sometimes called the "full table") contains over 950,000 IPv4 prefixes and 200,000+ IPv6 prefixes as of 2025. Every internet-connected router must process updates to this table in real time.

More specific prefixes (longer prefix lengths) take precedence over less specific ones. An announcement for 1.2.3.0/24 beats 1.2.0.0/16 for traffic to 1.2.3.x, because /24 is more specific.

// Famous ASNs

// How to Look Up an ASN

You can find the ASN for any IP address using ip-tracker.eu — it displays the ASN, organisation name, prefix, RIR, and routing information for any IP address.

From the command line:

Online tools include RIPE NCC's RIPEstat (stat.ripe.net), Hurricane Electric's BGP toolkit (bgp.he.net), and the global routing table viewer at bgpview.io.

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