The issue isn't about the remote host "knowing" about your network layout, the issue is that it makes it very easy for a target machine to DoS your system, since each accept() takes up one socket descriptor, a limited resources. AFAIK Windows only accepts about ~256 open descriptors at a time? Something really low. It would be pretty easy for someone to abuse that, even if the connections are opened for a few hundred ms.
Of course, mIRC is not meant to have a robust socket implementation, DoS concerns are better left to your OS or router's firewalls.
