From what you have said makes some sense to me. The best way to solve this is to consult the user manual or belkin's website. I know that it has something to do with NAT (Network Address Translation) that is enabled within your router and needs to be set up using a trigger port that the application is using then entering public ports associated to the trigger port. Anyway i'm not that clued up on it yet but if u try the manual or website you may be able to get more help there looking for key words such as Port Mapping, Port Redirecting, Port Forwarding or something like that.
For some reason i assume that you are on about dc++ (the application) with regards to active and passive modes if that is the case let me know and i'll add a txt file that has instructions on how to do it (well sort of still havent worked it out myself yet).