Uses

VLANs are created to provide the segmentation services traditionally provided by routers in LAN configurations. VLANs address issues such as scalability, security, and network management. Routers in VLAN topologies provide broadcast filtering, security, address summarization, and traffic flow management. By definition, switches may not bridge IP traffic between VLANs as it would violate the integrity of the VLAN broadcast domain.

This is also useful if someone wants to create multiple Layer 3 networks on the same Layer 2 switch. For example if a DHCP server (which will broadcast its presence) were plugged into a switch it would serve anyone on that switch that was configured to do so. By using VLANs you easily split the network up so some hosts won't use that server and default to Link-local addresses.

Virtual LANs are essentially Layer 2 constructs, compared with IP subnets which are Layer 3 constructs. In an environment employing VLANs, a one-to-one relationship often exists between VLANs and IP subnets, although it is possible to have multiple subnets on one VLAN or have one subnet spread across multiple VLANs. Virtual LANs and IP subnets provide independent Layer 2 and Layer 3 constructs that map to one another and this correspondence is useful during the network design process.

By using VLAN, one can control traffic patterns and react quickly to relocations. VLANs provide the flexibility to adapt to changes in network requirements and allow for simplified administration.