|  | 9.8 | The Benefits of Virtual LANs (VLANs)
| 9.8.1 | Layer 2 broadcast control
Virtual LANs or VLANs are used for several reasons including the creation of separate broadcast domains within a switched network. We will see that routers are necessary to pass information between different VLANs. A VLAN can be thought of as a subnetwork. There are several ways to implement VLANs, but one of the most common methods is to separate subnetworks into separate VLANs. It is important to note that VLANs are not necessary to have separate subnetworks on a switched network, but as you will see they provide more advantages when it comes to things like data link (layer 2) broadcasts. Without VLANs, a layer 2 broadcast such as anARP Request would be seen by all hosts on the switched network. On a large switched network, these ARP Requests can consume unnecessary network bandwidth and host processing cycles. Normally, only routers would stop the propagation of layer 2 broadcast such as these ARP Requests. Notice in the Figure, that the hosts are on different subnetworks, yet the ARP Request is being received by all of the computers. This can be especially an issue in a network with Windows 95/98 computers. Windows 95/98 computers keep entries in their ARP tables for only 120 seconds. This means, that if the ARP table hasn’t communicated with a device for 120 seconds, it will erase its IP address - MAC address mapping from the ARP table. The next time the host needs to communicate with this same device, it will need to do another ARP Request. UNIX computers on the other hand normally keep entries in their ARP tables for about 20 minutes.