The Cisco 1721 supports standards-based IEEE 802.1Q VLAN routing, which enables enterprises to set up multiple VLANs and route between them for added security within the internal corporate network. So you should be able to perform what Ianh stated.
Create an 802.1q trunked port on the switch connecting to the router using command
Switch# interface 21
Switch# switchport mode multi vlan1,2
On your router
leve the original interface like this:
interface FastEthernet0
no ip address
speed auto
Then create a sub-interface by issuing the interface command in global config mode like this:
Router(config)# interface FastEthernet0.16
Next set the encapsulation to dot1q and whatever your vlan is as well.
encapsulation dot1Q 1
Then set the ip address
ip address X.X.X.X
Perform the same thing with the next sub-interface
interface FastEthernet0.17
encapsulation dot1Q 2
ip address X.X.X.X
NOTE however many VLans you are routing is how many sub-interfaces you will need. Basically I just drew out what Ianh said in a sligh bit more detail.