Chapter 11 IPsec VPN for FortiOS 5.0 : Dynamic DNS configuration : Dynamic DNS over VPN concepts
  
Dynamic DNS over VPN concepts
A typical computer has a static IP address and one or more DNS servers to resolve fully qualified domain names (FQDN) into IP addresses. A domain name assigned to this computer is resolved by any DNS server having an entry for the domain name and its static IP address. The IP address never changes or changes only rarely so the DNS server can reliably say it has the correct address for that domain all the time.