What is a VLAN and what is the purpose of 802.1Q tagging?

• A. A VLAN is a physical switch.
• B. VLAN tagging is used to encrypt traffic.
• C. A VLAN segments broadcast domains; 802.1Q tagging carries multiple VLAN IDs on a single trunk link.
• D. 802.1Q tagging is optional and does not affect frame structure.

Answer: (C)

VLANs create separate broadcast domains within a single physical network, so devices in different VLANs don’t receive each other’s broadcasts even though they may share the same switches. The purpose of 802.1Q tagging is to let multiple VLANs share one physical link, known as a trunk. It does this by inserting a small tag into Ethernet frames that identifies the VLAN ID the frame belongs to. Switches read this tag on trunks and forward frames to the correct VLAN, preserving isolation between VLANs. On access ports, frames are untagged and assigned to a single VLAN; on trunk ports, frames are carried with tags. So the tagging changes the frame structure to include the VLAN ID and enables carrying multiple VLANs over one link.