Port 80, 1935, 40000, 40001, 40002 , 40005 must be free in the machine on which you intend to install Dimdim Web Meeting Server.
The Dimdim Servers use ports 80 &1935 for external communication. But port 80 alone is enough to be opened for external communication.
In addition, ports 40000/ 40001/ 40002 / 40003 / 40005 must be free for internal communication