amp and nat?

hello I am trying to get fwd and amp set back up I am behind nat my friend 
is running sipura he isn't able to connect iI set my sip.conf as follows
externip=hanksmith.mine.nu
localnet=192.168.0.4/255.255.255.0
the 192.168.0.4 is the ip that my linux box is on the hanksmith.mine.nu is 
the domain that is pointing to my public ip address
am I doing anything wrong here?
thanks
hank




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