VLAN

VLAN technology can be used to separate multiple devices operating on the same physical network into completely separate networks without the devices ever seeing one another. A typical use case involve two departments separated from each other but each using the same infrastructure, such as a shared switch or router. Only devices using the same VLAN ID would be able to see each other.

For this to happen, 4 bytes are added in front of the packet-type field in the Ethernet frame, which pushes the original packet-type field back by 4 bytes. The Ethernet frame will still have a maximum length of 1518 bytes (including CRC), which means that instead of a maximum of 1500 transferrable bytes, the transferrable amount of bytes per Ethernet frame will shrink to 1496 bytes per packet. VLAN-tagged packets are typically forwarded by any switch since the type field has simply been replaced, and in most cases, only the destination MAC, source MAC, and packet type is checked. In this case, the packet is simply an unknown protocol and will be forwarded by the switch.