What is an ASN?

The internet is made up of thousands of independent networks. ASNs are how they identify each other.

ASN Explained

An ASN (Autonomous System Number) is a unique identifier assigned to a network or group of networks under common administrative control. Each ASN represents an "autonomous system"—basically, a network that has its own routing policy.

When you look up an IP address, you'll often see something like AS15169 Google LLC. That AS15169 is Google's autonomous system number.

Why ASNs Exist

The internet isn't one big network—it's thousands of smaller networks connected together. Your home ISP is one network. Google is another. Amazon is another. Universities, governments, and major corporations often have their own.

For these networks to exchange traffic, they need a way to identify each other. That's what ASNs do. They're like company IDs that routers use to decide how to forward data packets.

How Routing Works

Networks with ASNs communicate using BGP (Border Gateway Protocol). When your ISP needs to send traffic to Google's servers, BGP figures out the path:

1. Your ISP's network (its own ASN)
2. Possibly one or more transit providers (each with their own ASN)
3. Google's network (AS15169)

This chain of ASNs is called an "AS path." Routers use these paths to determine the best route for traffic.

Who Gets an ASN?

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

To get an ASN, an organization must demonstrate they need to connect to multiple other networks with independent routing policies. Most small businesses don't need one—they just use their ISP's network.

ASN Formats

Originally, ASNs were 16-bit numbers (1 to 65,535). As the internet grew, we ran low, so 32-bit ASNs were introduced (up to 4,294,967,295).

You'll see them written as simple numbers like AS15169 or in "asdot" notation for larger numbers like AS4.5.

What ASNs Tell You

When you look up an IP address, the ASN reveals:

• Who operates the network – The company or organization
• Network type – ISP, cloud provider, enterprise, university, etc.
• Geographic focus – Some ASNs serve specific regions

For example, if an IP belongs to AS16509 Amazon.com, Inc., you know it's part of Amazon's infrastructure (likely AWS).

Common ASNs You'll See