ASN (Autonomous System Number)

What is an ASN?

An Autonomous System Number (ASN) is a unique identifier assigned to a network—like an ISP, university, or large company—that manages its own routing policy on the internet. Think of it as a "network ID card" that lets other networks know who they're talking to.

Every major ISP has at least one ASN. For example, Comcast uses ASN 7922, AT&T uses 7018, and Verizon uses 701. When you connect to the internet, your traffic is routed through your ISP's autonomous system before reaching its destination.

Why ASNs exist

The internet is made up of thousands of independent networks that need to exchange traffic with each other. ASNs provide a standard way for these networks to identify themselves and negotiate routing.

When your data travels across the internet, it typically passes through multiple autonomous systems. Your ISP (one AS) hands your traffic to a transit provider (another AS), which might pass it to a content delivery network (yet another AS) before it reaches the website you're visiting.

What your ISP's ASN tells you

Seeing your ISP's ASN in Network Weather confirms which network is providing your internet access. This can be useful for:

• Verifying your connection: If you're paying for Comcast but see a different ASN, something might be misconfigured
• Understanding routing: Your traffic's path through the internet depends on how your ISP's AS connects to others
• Troubleshooting: If you report network issues to your ISP, knowing your ASN helps their support team investigate

Well-known ASNs

Some ASNs you might encounter:

Looking up ASN information

If you want to learn more about a specific ASN, tools like bgp.tools and PeeringDB provide detailed information about network ownership, peering relationships, and geographic coverage.

Network Weather automatically looks up your ISP's ASN and displays it alongside other connection details, so you always know which network is serving your internet connection.