IV VLANs Creating Virtual LANs
A VLAN, or Virtual LAN, is a logical subnet created through the configuration of networking switches. This VLAN may be part of a larger LAN, or even part of WAN, spanning components in different cities. As seen above when discussing LANs, devices on a LAN may be grouped into physical subnets by locating the devices along the same cable, on the same network hub, etc. This typically implies that the devices in the subnet are connected by the same, or at least very similar, media (such as 10BaseX or 100BaseX, which will be discussed later in this text), and that a device on a high-speed fiber backbone wont be in the same subnet as a workgroup server attached to a local hub in a department wiring closet.
One advantage of subnetting in general is that certain broadcast or multicast traffic is transmitted only once by the sender, but can be received by all the workstations on the destination subnet. Additionally, subnets allow you to optimize the traffic of users who communicate very frequently with each other (suppose Mary and Joe are always sharing large engineering simulation data files), so that these users experience the best possible network communication speeds while good network performance on your LAN is preserved.
What if certain machines in your LAN or WAN, which are not on the same physical subnet, would benefit from the kind of network communication optimization provided by subnets? Thats where VLANs come in. With a VLAN, you can get the benefits of subnets without the physical proximity and similarity required for directly-connected physical subnets. Switches and other network devices can be configured to allow data that would not normally be passed between physical subnets to be shared among multiple physical subnets, regardless of their location, creating a virtual LAN link, or VLAN. In effect, putting together a VLAN amounts to running software that simulates a virtual patch panel, allowing you to connect (patch together) various devices around your network into a logical subnet, in a configuration that is most convenient for your needs. Also importantly, it allows you to change the configuration of your VLAN as your needs change, without resorting to moving or rearranging your hardware resources. (For example, suppose Mary is transferred across town, but still needs to communicate with Joe. If that cross-town location can be included in the VLAN, you may be able to adapt your network to Marys new location by bringing up a control program on your server and changing settings in software, rather than by physically accessing hardware.)
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