Chapter 1 What’s New for FortiOS 5.2.1 : IPv6 : IPv6 Address Ranges
  
IPv6 Address Ranges
IPv6 address ranges can now be created, using either the web-based manager or the CLI.
Syntax
config firewall address6
edit <name>
set type iprange
set start-ip <address>
set end-ip <address>
end
end