Man, it is quite simple, You have given public IP on your private network, as this IP is not on your private network so it is not going to conflict. On the other hand your linux box is connected to internet , so if any computer connected directly to internet using that ip can cause conflict but that is not possible as only your ISP can assign that IP to any computer and it is not going to give same IP to two computer..
BTW. how you got ISDN 256 ? I think we have max ISDN speed of 128 and that of dual channel ?