First, an IP address doesn't automatically identify a criminal suspect. It's just a unique address for a device connected to the Internet, much like a street address identifies a building. In most cases, an IP address will identify a router that one or more computers use to connect to the Internet.

But in many situations, an IP address isn't personally identifying at all. When it traces back to a router that connects to many computers at a library, cafe, university, or to an open wireless network, VPN or Tor exit relay used by any number of people, an IP address alone doesn't identify the sender of a specific message. And because of pervasive problems like botnets and malware, suspect IP addresses increasingly turn out to be mere stepping stones for the person actually "using" the computer—a person who is nowhere nearby.