Why VLANs?

Many people will tell you VLANs are so darn virtual that you tend to lose them for no apparent reason. I'll save that discussion for the “Shooting Trouble with VLANs” section and the Trouble Tickets. To get a handle on VLANs, I want you to think about the function of a router. Physically each interface or wire is a broadcast domain, but it is more often referred to as a subnet. Traffic from one router can pass from one local interface to another because the router knows about its directly connected networks. Interrouter communications occur because of not only physical components but also because of routed and routing protocols.

Switch broadcast domains are called VLANs. By default all ports on a switch belong to VLAN1, as you can ...

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