With VLAN tagging, a tagging data field is inserted into the Ethernet frame to identify the individual VLANs. This consists of the Tag Protocol Identifier( TPID) and the Tag Control Information( TCI). One data field in the 16-bit TCI data field is the 12-bit VLAN Identifier (VID).
The VID data field indicates which virtual network the frame belongs to. If the VID data field has a 0, then it does not belong to any virtual network. In such a case, the Priority Code Point( PCP), a 3-bit data field in the Tag Control Information (TCI), merely determines the priority with which the data packet is transmitted in the network. All hexadecimal values can be used for the VLAN identification (VID), only the value 0xFFFis reserved. With 12 bits this means that a total of 4095 VLANs can be identified.