tag protocol identifier (data field) (TPID)
With VLAN tagging according to IEEE 802.1Q, the information required for setting up a corresponding virtual LAN (VLAN) is accommodated in a data field, the VLAN tag field.
This 4- byte data field is inserted into the Ethernet frame between the Source MAC Address data field and the Ethernet Type data field. The 4 byte total field consists of the 16 bit data field for the Tag Protocol Identifier (TPID) and around the also 16 bit data field for the Tag Control Information( TCI). The TPID data field is also referred to as Ethertype 802.1Q and is set to the value 0x8100 for VLANs. This value is reserved for VLAN applications.