If you want to play Settlers 7 and be online (at a lanparty for example) with more than 1 player at the same time, you have to add an option to the Options.ini used by Settlers7. This file can be found in your My Documents folder:
XP: C:\Documents and Settings\YourName\My Documents\Settlers7\Options.ini
Vista\7: C:\Users\YourName\My Documents\Settlers7\Options.ini
Add the following lines to the ini File:
DefaultPort = 9205
Every player should use a different port for this to work.