Chapter 10 Install and System Administration for FortiOS 5.0 : VLANs : VLANs in NAT mode : Adding VLAN subinterfaces : Physical interface
  
Physical interface
The term VLAN subinterface correctly implies the VLAN interface is not a complete interface by itself. You add a VLAN subinterface to the physical interface that receives VLAN-tagged packets. The physical interface can belong to a different VDOM than the VLAN, but it must be connected to a network router that is configured for this VLAN. Without that router, the VLAN will not be connected to the network, and VLAN traffic will not be able to access this interface. The traffic on the VLAN is separate from any other traffic on the physical interface.
When you are working with interfaces on your FortiGate unit, use the Column Settings on the Interface display to make sure the information you need is displayed. When working with VLANs, it is useful to position the VLAN ID column close to the IP address. If you are working with VDOMs, including the Virtual Domain column as well will help you troubleshoot problems more quickly.
To view the Interface display, go to System > Network > Interface.
See Also
Adding VLAN subinterfaces
IP address and netmask
VLAN ID
VDOM
Configuring security policies
Configuring routing