VLAN and Subinterfaces

i want to know the procedure for creating VLAN and Subinterface on linux box... actully i know about both vlan and subinterfaces as well as trunking but i am not able to understand command in linux...

 

i want to know what the following commands do exectly...

 

vconfig add eth0 3

ifconfig eth0:3

actully i want to know the difference between this two...

 

and let say i have two subinterfaces created on 3 and 4 ...and my linux box is connected with cisco switch with trunk ;having vlan 3 and 4...

now i want to establish communication between host on vlan 3 and host on vlan 4 using linux box ... my cisco switch is layer-2 switch...

 

subinterface 3 on linux box and host in vlan 3 which connected with the switch both are in same subnet...(192.168.1.0 )

subinterface4 on linux box and host in vlan 4 which connected with the switch both are in same subnet...(192.168.2.0 )

 

now i have to enable intervlan routing between subinterface3 and subinterface4... to full fill my requirement...or routing between this subinterface will be enable by default...