Virtual LANs

To understand VLANs, it is first necessary to have a basic understanding of how a traditional LAN operates. A standard local area network (LAN) uses hardware such as hubs, bridges, and switches in the same physical segment to provide a connection point for all end node devices. All network nodes are capable of communicating with each other without the need for a router; however, communications with devices on other LAN segments does require the use of a router.

As a network grows, routers are used to expand the network. The routers provide the capability to connect separate LANs and to isolate users into broadcast and collision domains. Using routers to route data around the network and between segments increases latency. Latency refers to delays in transmission caused by the routing process.

Virtual LANs (VLANs) provide an alternate method to segment a network and in the process, significantly increase the performance capability of the network, and remove potential performance bottlenecks. A VLAN is a group of computers that are connected and act as if they are on their own physical network segments, even though they might not be. For instance, suppose that you work in a three-story building in which the advertising employees are spread over all three floors. A VLAN can let all the advertising personnel use the network resources as if they were connected on the same segment. This virtual segment can be isolated from other network segments. In effect, it would appear to the advertising group that they were on a network by themselves.

VLANs offer some clear advantages. Being able to create logical segmentation of a network gives administrators flexibility beyond the restrictions of the physical network design and cable infrastructure. VLANs allow for easier administration because the network can be divided into well-organized sections. Further, you can increase security by isolating certain network segments from others. For instance, you can segment the marketing personnel from finance or the administrators from the students. VLANs can ease the burden on overworked routers and reduce broadcast storms. Table 1 summarizes the benefits of VLANs.

Table 1 Benefits of VLANs

Advantages	Description
Increased security	By creating logical (virtual) boundaries, network segments can be isolated.
Increased performance	By reducing broadcast traffic throughout the network, VLANs free up bandwidth.
Organization	Network users and resources that are linked and communicate frequently can be grouped together in a VLAN.
Simplified administration	With a VLAN, the network administrator's job is easier when moving users between LAN segments, recabling, addressing new stations, and reconfiguring hubs and routers.