I've managed to spoof the Switch with a false MAC address.

I 've changed my MAC addresst (my terminal computer) to the default Gateway computer (win 2k server ). In the bridge table it show one MAC address in 2 port (the port to my box and to the server box). And other client sometime suffer from accessing the default-gateway.

However. when i changed the IP of my box to the server IP, it states a IP confict and the IP turns to 0.0.0.0 (phew, luckily). I thought .... if some one manage to successfully change the IP, it would be messy for whole the network.