Chapter 11 IPsec VPN for FortiOS 5.0 : Hub-and-spoke configurations : Configuration overview : Hub-and-spoke infrastructure requirements
  
Hub-and-spoke infrastructure requirements
The FortiGate hub must be operating in NAT mode and have a static public IP address.
Spokes may have static IP addresses, dynamic IP addresses (see “FortiGate dialup-client configurations”), or static domain names and dynamic IP addresses (see “Dynamic DNS configuration”).