Okay If i understand you correctly

You have an app thats running on a computer connected to the net! This app is a server app that you have clients connecting to

Your problem is that the server apps ip number keeps changing!

If that is the case then the only way you are going to resolve this without emailing everycompter that uses your client with each ip adress change

Is to get yourself a static ip. Even if you are using cable (dsl) unless you have requested a static IP address your IP will change every couple of weeks

If you are using a dial up modem your ip will change each time you dial up.