VLAN

VLAN (Virtual Local Area Network) is a virtual local area network. With its help it is possible to unite several networks. Thanks to it it is possible to create additional virtual networks to already existing physical network devices.

Principle of operation

The main purpose of using VLANs is to create a number of subnets to separate multiple devices within the operation of a single switch. This separation is often required in organizations where there is a need to subdivide several departments into subnets, such as HR, accounting, and IT. Ethernet frames are isolated from each other. Such separation is useful first of all for data security of a particular department.

Each Ethernet frame must contain a tag in the subheader, thanks to which the common switch determines who owns a particular subnet. The 4-byte tags must include 2 mandatory fields:

1. TPID with protocol information;
2. TCI with control information, which is divided into several field-blocks:
   • PCP with indication of priority traffic;
   • CFI with MAC address formats;
   • VID with virtual network identifier;