VPI
Stands for "Virtual Path Identifier." The VPI is an 8-bit header inside each ATM cell that indicates where the cell should be routed. ATM, or asynchronous transfer mode, is a method of sending data in small packets of fixed sizes. It is used by many ISPs (Internet Service Providers) for transferring data to client computers.
As an ATM cell moves across a network, it typically passes through several ATM switches. The VPI tells the switches where to route the packet of information, or what path to take. Hence the name, "virtual path identifier." The VPI is used in conjunction with the VCI, or virtual channel identifier.