This thread is more about being unable to connect to a network by way of their servers. Your issue appears to be more related to a known dcc prob when you are connected on port 6667. That isnt related to mIRC, but is due to some routers having other uses for 6667 and causing some probs with dcc. (see cant dcc thread
mIRC is software used to chat. It has nothing at all to do with any networks or servers. The choice of what ports they decide to use is totally up to them and their circumstances. We cant do anything about it.