Technology: Network
Area: Configuration General
Vendor: Cisco
Software: 12.X, 15.X
Platform: ISR Routers, ASR Routers

Sub-interfaces divide the parent interface into two or more virtual interfaces on which you can assign unique Layer 3 parameters such as IP addresses and dynamic routing protocols parameters. The IP address for each sub-interface should be in a different subnet from any other sub-interface on the parent interface (if you are not using VRF-Lite feature).

Switch# configure terminal
switch(config)# interface ethernet 2/1.1
switch(config-if)# ip address 192.0.4.1 255.0.0.0
switch(config-if)# encapsulation dot1Q 66

Because sub-interfaces work always on trunk links, there is a necessity of encapsulation definition. The encapsulation command defines the VLAN tag (VLAN number) which will be using by this sub-interface.