Using a 2600 series router and only one LAN interface, you need a "Plus" version of IOS in order to create subinterfaces on it. (Which may require more memory and flash in your router.) Then, configure the switch port the router plugs into to be an 802.1Q trunk port.
Here's an example of a configuration I have working on a 2620 router:
interface FastEthernet0/0
no ip address
duplex auto
speed auto
!
interface FastEthernet0/0.1
encapsulation dot1Q 1 native
ip address 10.1.0.1 255.255.0.0
!
interface FastEthernet0/0.5
encapsulation dot1Q 5
ip address 10.5.0.1 255.255.0.0
!
interface FastEthernet0/0.16
encapsulation dot1Q 16
ip address 172.16.0.1 255.255.0.0
!
interface FastEthernet0/0.17
encapsulation dot1Q 17
ip address 172.17.0.1 255.255.0.0
!
The subinterface number (0/0.xxx) is arbitrary; I chose to match the "xxx" with the VLAN number on the encapsulation dot1Q xxx".
Whatever is the native VLAN defined on your switch port, that subinterface should also have it indicated with the "native" command line option. (See my 0/0.1 subint. for example.)
It's a one-armed router; but it works better than nothing, especially if you have other protocols to route besides TCP/IP. However, if all you have to route is IP, then inter-VLAN routing performance would be greatly improved if you had a 3550 series switch, even one with the SMI software. That's because you can activate Layer 3 switching of IP traffic on those, and define VLAN interfaces like this:
interface vlan1
ip address 10.1.0.1 255.255.0.0
!
interface vlan5
ip address 10.5.0.1 255.255.0.0
!
interface vlan16
ip address 172.16.0.1 255.255.0.0
!
interface vlan17
ip address 172.17.0.1 255.255.0.0
!
Hope this helps.
