It's hard to answer without knowing what you are doing. What's the difference between the server on port A and the one on port B? Or rather, why would someone want to connect to the server on port A as opposed to one on port B?

Generically speaking, the problem is the same as finding a someone's phone number. How do you that? Maybe you search a directory. Maybe you ask someone who knows. In both of these, you query an external system.

Therefore, a possible solution is to have the server communicate it's vitals to a directory server. The client (user or program) then selects the server to which it wishes to connect from the directory.

For example, ICQ used to work this way (before changes were made to fight spam and privacy invasion). The ICQ client would become a server and then connect to the central ICQ server. Someone wishing to send you a message would query the central ICQ server for your IP address.

Another example is battle.net. People wishing to play Starcraft over the internet log into the battle.net chat server (through the game). When someone creates (hosts) a game, Starcraft contacts battle.net letting it know at which IP the game resides. battle.net would present a list of running games to users and passes on the IP address and port to people wanting to join.

Update: Added examples.