If you forward port 113 to your pc, then all mIRC pc's behind your router will all have their identd handshake talking to your pc. So, if you check the box in mIRC-options/identd/enable-only-when-connecting, then your pc will have identd enabled, but everyone else will be disabled. If that box isn't checked, then everyone else gets your same identd value.
I tried leaving that box unchecked, and then changing my identd value after i connected, but i've found that it doesn't work right if a 2nd mirc connects soon after the 'main' mIRC owning identd changes the value. However, if you wait long enough, you can then control whatever identd value the other pc's get.
They can avoid being given your identd value only by disabling identd in their own pc, which almost always adds the "~" to their address. However, you must be careful to use address masks that discern the difference between someone who has identd and one who doesn't. ie, $address($nick,3) does *!*userid@isp because it's trying to match someone whether they have identd or not.
