Unfortunately I use Mac so I can't use exe-files.Perhaps I
can find a similar program for Mac.
I don't quite understand what you're talking about.
If I understand right I must have a static ip-adress to
avoid being firewalled.
You have to have a static IP address if you're forwarding your ports manually. That is because you have to tell which IP you're forwarding that port to, and that is impossible if your IP changes randomly. But note, that you don't need a global static IP, you only have to tell your router to assign always the same local IP address to your box (static local IP).
In practice, whenever another client wants to connect to you, it will make a request to your global IP address - which actually is your router. By default, your router doesn't know what to do with that request and rejects it. Well, unless the requested port is forwarded. In this case your router will forward the request on its LAN to the IP specified in the port-forward entry - which should be your box, and now that will be able to handle the request.
