9.3 Inter-VLAN Routing Overview  
  9.3.5 Dividing physical interfaces into subinterfaces  
A subinterface is a logical interface within a physical interface, such as the Fast Ethernet interface on a router.

Multiple subinterfaces can exist on a single physical interface.

Each subinterface supports one VLAN, and is assigned one IP address. In order for multiple devices on the same VLAN to communicate, the IP addresses of all meshed subinterfaces must be on the same network or subnetwork. For example, if subinterface 2 has an IP address of 192.168.1.1 then 192.168.1.2, 192.168.1.3, and 192.1.1.4 are the IP addresses of devices attached to subinterface 2.

In order to route between VLANs with subinterfaces, a subinterface must be created for each VLAN.

The next section discusses the commands necessary to create subinterfaces and apply a trunking protocol and an IP address to each subinterface.