The IEEE committee that worked on VLANs took advantage of the fact that a new header was about to be added to MAC frames. The tag header actually is used for triple duty:
It contains a 12-bit VLAN identifier.
It includes a 3-bit priority value.
Some LANs are made up Ethernets, Token Rings, and FDDI rings that are connected by bridges that translate between frame formats. A tag header can include information that assists in this translation.
VLAN identifiers are 12-bit quantities. Hence, they can represent decimal numbers ranging from 0 to 4095. Some of these values are reserved:
0 does not correspond to a VLAN. It is called the null VLAN ID and is used in a priority-tagged frame to indicate that no ...