Section 9. Implementing VLANs and Trunks
Switches operate on Layer 2 of the OSI model and routers operate on Layer 3. Switches and routers increase the number of collision domains, decreasing the amount of collisions in a network; routers also increase the number of broadcast domains while reducing their size.
Prior to the introduction of VLANs, network designers segmented large Layer 2 networks with routers. A router would be installed between the two networks. All data communication between the two would pass through the router, and the broadcast domain was limited to the physical location of the network.
VLANs introduced a way to logically segment a broadcast domain within a switch. Additionally, VLANs are not bound to a physical location. ...