CCNA 3.4: Configure, verify and troubleshoot inter-VLAN routing
VLANs divide broadcast domains in a LAN environment. Inter-VLAN routing is necessary when a host in one VLAN needs to communicate with a host in another VLAN because the packets must be routed between the VLANs. On Catalyst switches it is accomplished by the creation of Layer 3 interfaces called switch virtual interfaces (SVIs). Router on a stick is a way to reach multiple VLANs over a single physical interface by way of configuring subinterfaces.