The public IP address is the one I get for my ADSL modem, from my provider? (I suppose)
OKAY... So this kind of setup is safe:
ADSL modem -> router -> workstations (each one setup with a firewall which block everything except 192.168.1.0/24 and 127.0.0.1)
... i've check at grc.com for sheildsup... and I saw that my connexion is done via a 2xx.x.x.x address... So, no 192.x.x.x IPs seems to travel over the web...
That's good news...!