The Virtual Path Identifier (VPI) is an identification number for the logical connection hierarchy, the virtual path (VP), in B-ISDN
. The VPI field is an 8-bit long data field in the header of the ATM cell at the User Network Interface (UNI) or a 12-bit long data field at the Network to Network Interface (NNI) and at B-ICI. Together with the 16-bit Virtual Channel Identifier (VCI), the Payload Type (PT) and the Cell Loss Priority (CLP) and Header Error Control
(HEC) data fields, it forms the NNI header.segment type (ST) and the message identifier (MID) are used.