Chapter 10 Install and System Administration for FortiOS 5.0 : Advanced concepts : DHCP servers and relays : DHCP in IPv6
  
DHCP in IPv6
You can use DHCP with IPv6 using the CLI. To configure DHCP, ensure IPv6 is enabled by going to System > Admin > Settings and enable IPv6. Use the CLI command
config system dhcp6.
For more information on the configuration options, see the CLI Reference.
See Also
Service
DHCP options
DHCP Monitor
Breaking a address lease
DHCP servers and relays