Asterisk register problem


i found a strange problem,

i have a asterisk behind a nat local ip is:
extern ip
if i try to connect from home or some where else where the local subnet is 192, then register fails
if i try to register from a subnet something like then everything works fine audio and calls and everything. does anybody got a sulution for this??? or only way to change the server’s subnet to a subnet which is not used anywher in the world:)

Thank SzaZo