Network Warrior

Chapter 4. VLANs

Virtual LANs, or VLANs, are virtual separations within a switch that provide distinct logical LANs that each behave as if they were configured on a separate physical switch. Before the introduction of VLANs, one switch could serve only one LAN. VLANs enabled a single switch to serve multiple LANs. Assuming no vulnerabilities exist in the switch's operating system, there is no way for a frame that originates on one VLAN to make its way to another.

Connecting VLANs

Figure 4-1 shows a switch with multiple VLANs. The VLANs have been numbered 10, 20, 30, and 40. In general, VLANs can be named or numbered; Cisco's implementation uses numbers to identify VLANs by default. The default VLAN is numbered 1. If you plug a number of devices into a switch without assigning its ports to specific VLANs, all the devices will be in VLAN 1.

Figure 4-1. VLANs on a switch

Frames cannot leave the VLANs from which they originate. This means that in the example configuration, Jack can communicate with Jill, and Bill can communicate with Ted, but Bill and Ted cannot communicate with Jack or Jill in any way.

For a packet on a layer-2 switch to cross from one VLAN to another, an outside router must be attached to each of the VLANs to be routed. Figure 4-2 shows an external router connecting VLAN 20 with VLAN 40. Assuming a proper configuration on the router, Bill will now be able to communicate ...