Chapter 11 IPsec VPN for FortiOS 5.0 : IPv6 Configuration : IPv6 IPsec VPN : Overview of IPv6 IPsec support : Certificates
  
Certificates
On a VPN with IPv6 phase 1 configuration, you can authenticate using VPN certificates in which the common name (cn) is an IPv6 address. The cn-type keyword of the user peer command has an option, ipv6, to support this.