Ethernet technology implements data communication over shared media based on carrier sense multiple access with collision detection (CSMA/CD). If there are a large number of PCs on the Ethernet, security risks and broadcast storms may occur, deteriorating network performance and even causing network breakdowns. The virtual local area network (VLAN) technology is therefore introduced to solve the preceding problem. This note describes basic VLAN principles, working principles of different Layer 2 interfaces, VLAN applications, data forwarding principles, and basic VLAN configuration methods.