Each VLAN (virtual LAN) is a separate broadcast domain and IP subnet, so hosts in different VLANs cannot talk directly. Something must route between them, and that device becomes each VLAN's default gateway. The CCNA covers two methods: router-on-a-stick with subinterfaces, and switch virtual interfaces (SVIs) on a multilayer switch. A third, legacy method uses one physical router interface per VLAN, which does not scale because you run out of ports.
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