If you use private IP addresses within
your internal networks, you must use NAT to translate the private
addresses to public addresses that can be routed on external networks.
If you define Layer 3 interfaces on the firewall, you can
configure a Network Address Translation
(NAT) policy to specify whether source or destination IP
addresses and ports are converted between public and private addresses
and ports. For example, private source addresses can be translated
to public addresses on traffic sent from an internal (trusted) zone
to a public (untrusted) zone. NAT is also supported on virtual wire
interfaces.