What is a VLAN?
Why Interviewers Ask This
Interviewers use this question to quickly assess whether a candidate has the foundational knowledge required for Networking development. It reveals whether you understand the building blocks that more complex concepts rely on.
Answer
A VLAN (Virtual Local Area Network) is a logical segmentation of a physical network into separate broadcast domains, as if they were completely separate physical networks. VLANs are configured on managed switches. Devices in different VLANs cannot communicate without going through a router (or Layer 3 switch). Benefits: security (separate sensitive traffic — e.g., management VLAN from user VLAN), traffic management (reduce broadcast domain size), flexibility (group devices by function regardless of physical location), and cost (one physical switch replaces multiple). VLAN tagging uses IEEE 802.1Q — a 4-byte tag is added to Ethernet frames to identify which VLAN the traffic belongs to. Trunk ports carry traffic from multiple VLANs between switches; access ports connect to end devices and belong to a single VLAN.
Pro Tip
If you're unsure about a detail, say so honestly and explain your reasoning. Interviewers respect candidates who can think through uncertainty rather than bluffing.