Hi all, at least i've solved my problem with NAT, thanks to all for the 
answers.
Now i'm facing another problem. I've currently set up a VPN betwenn our 
two offices with two Monowall and all works fine.
But i've got a problem with our programmers. They needs to connect in 
VPN from a third office that should not enter into the VPN.
I try to explain better: this office should not enter into our VPN 
because its a customer. But we're doing a consultant for them and so 
when we go there we should connect with laptop to our network. I've 
tried to use the "mobile client VPN setup" but it doesn't work.
I think that kind of configuration works just if laptop connects 
directly to internet.
In this case we'va a router handling connections and doing DHCP stuff.
The two subnet are completly different.
Is there a way to establish a connection between a laptop behind a 
router and monowall? Have to say that monowall has a public static IP.

Thanks in advance,
    Alberto