Virtual path identifier
The 24-bit Address field in the ATM cell header is broken into two subfields: the 8-bit virtual path identifier (VPI) and the 16-bit virtual channel identifier (VCI). The combined 24 bits allow the unique identification of 16,777,215 virtual circuits per UNI. On NNIs, the VPI field is extended into the GFC field, which adds 4 bits to the range. Although this might seem like overkill, in reality the full address range is limited.
VPIs identify bundles of channels going to the same geographic or functional location. As such, the full range of VCIs is not used for each VPI. VCIs identify individual virtual circuits in the interface. The VPI/VCI combination must be unique for each virtual circuit on the interface. Like most other virtual circuit technologies, the VPI/VCI pair has local significance only, which implies that addresses can be reused for each individual interface.
Most devices have limitations on the number of virtual circuits and range of values to assign to the VPI and VCI. Certain VPI/VCI combinations are reserved for special use.