you would have to tell your router forward the port used by your remote administration program to the ip address
for instance vnc running as desktop 1 might have port 5801, reroute that port to machine a, then set vnc server on another to run on desktop 2 which would have a port of say 5802 and tell the router to forward port 5802 to machine b etc..
note this isnt very secure to have any form of remote admin visible to the internet
a better option would be to use a vpn software like hamachi (https://secure.logmein.com/products/hamachi/list.asp
) to create a tunnel from your remote site to your servers you want to administer, note this isnt 100% secure either but it is more secure