Allowing only 1 interface to connect to a network

[Pranav Lal <pranav.lal () gmail com>]

Hi all,

Suppose my computer is on a LAN. I have a modem connected to it. If I 
dial out via the modem, my computer will have 2 ip addresses; one 
from the LAN and 1 from the dialup modem. Is there any way to not 
allow such a simultaneous connection? So, if I am on the LAN and if I 
try and dial out, the computer blocks that feature? or the reverse, 
that is if I am  on connected dialup and  I connect to a LAN the LAN 
connection is not allowed?

At the moment I am looking at Windows xp, 2000 and 2003 server. 
However, I would also welcome tips on how to accomplish this in 
linux, solaris etc.

Pranav