Definition

Virtual Local Area Network (VLAN) technology divides a physical LAN into multiple broadcast domains, each of which is called a VLAN. Hosts within a VLAN can communicate with each other but cannot communicate directly with hosts in other VLANs. Therefore, broadcast packets are confined to within a single VLAN.

Purpose

Ethernet technology allows data communication over shared media through Carrier Sense Multiple Access/Collision Detection (CSMA/CD). When an Ethernet network has a large number of hosts, collision becomes a serious problem and can lead to broadcast storms. This can degrade network performance or even result a complete breakdown. Using switches to connect LANs can mitigate collisions, but cannot isolate broadcast packets or improve network quality.

VLAN technology divides a physical LAN into multiple VLANs to isolate broadcast domains. Hosts within a VLAN can only directly communicate with hosts in the same VLAN. They must use a router to communicate with hosts in other VLANs.

Figure 1 VLAN networking