|  | 9.8 | The Benefits of Virtual LANs (VLANs)
| 9.8.2 | Why use port-centric VLANs?
> The network in Figure , has a network address of 172.30.0.0. The network has been divided into subnetworks using the subnet mask 255.255.255.0. Therefore, two subnetworks exist on the network, 172.30.1.0/24 and 172.30.2.0/24. Question : What if you didn’t use VLANs, what would happen with a layer 2 broadcast, such as an ARP Request? Answer: All hosts, regardless of what subnetwork they belong to will receive the ARP request, as long as they are on the same switched network and there is no router between them and the ARP Request. Now you will see what happens if you use Port-centric VLANs. Port-centric VLANs are where the network administrator assigns each switch port to a specific VLAN as shown in Figure . It is important that the VLAN assignment on the switch matches the subnetwork assignment on the host that is connected to that switch port. For example, What would happen with ARP Requests on the network, but with the use of VLANs? Remember, the VLAN assignment is actually done at the switch. There is no VLAN assignment done on the individual hosts. Figure shows the VLAN numbers on the hosts for clarity purposes only. Host on the 172.30.1.0 subnetwork, the switch port will be configured as VLAN 1. Host on the 172.30.2.0 subnetwork, the switch port will be configured as VLAN 2. Notice that this time, only hosts on the same VLAN (VLAN 2), which are also on the same subnetwork, will receive the layer 2 broadcast, the ARP Request. Since layer 2 broadcast are only meant for hosts within the same subnetwork, the VLAN mapping on the switch keeps the unwanted, unnecessary layer 2 broadcast from being forwarded on links that do not need to receive those frames. It is important to note that it is the switch that is doing this filtering. This happened because the network administrator assigned the proper VLAN assignment to the proper switch port. The switch is configured as follows: Switch Port VLAN
> 1 | 1
> 2 | 2
> 3 | 1
> 4 | 2