4.1 VLAN Basics
4.1.2 Why are VLANs necessary?

In a legacy network, administrators assign users to networks based on geography. The administrator attaches the user's workstation to the nearest network cable. If the user belongs to the engineering department and sits next to someone from the accounting department, they both belong to the same network because they attach to the same cable. This creates some interesting network issues and highlights some of the reasons for using VLANs. VLANs help to resolve many of the problems associated with legacy network designs.

Network managers can logically group networks that span all major topologies, including high-speed technologies such as ATM, Fiber Distributed Data Interface (FDDI), and Fast Ethernet. By creating VLANs, system and network administrators can control traffic patterns, react quickly to relocations, and keep up with constant changes in the network due to moving requirements and node relocation. VLANs provide the flexibility to carry out these actions. The network administrator simply changes the VLAN member list in the switch configuration. The administrator can add, remove, or move devices or make other changes to the network configuration using software. The sections that follow examine the five issues listed in the Figure that warrant implementation of a VLAN.