interface fastethernet 0/0.1
encapsulation dot1q 1 native
 
interface fastethernet 0/0.21
encapsulation dot1q 21 <--vlan number
ip address 10.20.21.1 255.255.255.0 <--this will be the default gateway for your devices on vlan 21
 
interface fastethernet 0/0.20
encapsulation dot1q 20
ip address 10.20.20.1 255.255.255.0

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