Chapter 11 IPsec VPN for FortiOS 5.0 : IPv6 IPsec VPNs : Site-to-site IPv4 over IPv6 VPN example : Configure FortiGate A interfaces
  
Configure FortiGate A interfaces
Port 2 connects to the IPv6 public network and port 3 connects to the IPv4 LAN.
config system interface
edit port2
config ipv6
set ip6-address fec0::0001:209:0fff:fe83:25f2/64
end
next
edit port3
set 192.168.2.1/24
end