interface gigabitEthernet 0/1 switchport trunk encapsulation dot1q switchport mode trunk switchport trunk allowed vlan 10,20 no shutdown

Don't just assume it works. Run these from a test PC:

: Remember, you can only use the double colon once in an address to represent contiguous blocks of zeros.

interface fastEthernet 0/3 switchport mode access switchport access vlan 20 no shutdown