Chapter 11 IPsec VPN for FortiOS 5.0 : IPv6 Features : IPv6 Network Address Translation : NAT46
  
NAT46
NAT46 is used to translate IPv4 addresses to IPv6 addresses so that a client on an IPv4 network can communicate transparently with a server on an IPv6 network.
To enable NAT46, use the following CLI command:
config system nat46
set status enable
end
NAT46 policies
Security policies for NAT46 can be configured from the web-based manager. For these options to appear in the web-based manager, this feature must be enabled using Feature Select. You can then configure the policies under Policy > Policy > NAT46 Policy.
NAT46 policies and can also be configured from the CLI using the following command:
config firewall policy46