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What is an ASN?

The internet is made up of tens of thousands of separate networks, each operated by one or more different organizations, connected together. An ASN, or Autonomous System Number, is the unique identifier assigned to each of those networks.

Internet service providers (ISPs), universities, corporations, and cloud platforms each typically operate their own autonomous system. The ASN is what distinguishes each one from another. When data travels across the internet, it passes through multiple networks on its way from origin to destination. BGP, the routing protocol that manages this process, uses ASNs to identify which network is which and determine the best path, based on routing policies, for traffic to take.

For most people, ASNs are an invisible element of the internet’s architecture. But they show up in tools like IP lookup and enhanced traceroute services, where you might see which network owns a particular IP address or which networks your traffic is passing through. 

If you’ve ever noticed that your IP address is registered to your ISP rather than to you personally, that’s because the address is part of an IP range allocated to your ISP and announced through its autonomous system.

ASNs are also used to investigate traffic patterns, identify the source of suspicious activity, and understand how different parts of the internet are connected.