Using a Dynamic Domain Name Service (DDNS) means that users can reach your network by means of a domain name that remains constant even when its IP address changes. FortiOS has supported this feature in Network - DNS settings - Fortiguard DDNS service, which sounds great. Unfortunately, it does not work well in my home lab environment. My FortiGate is behind ISP modem and WAN port is using private ip address 192.168.20.2.
1. FortiGuard DDNS service
When use baisc FortiGuard DDNS settings wthout enabling 'Public IP Address", my WAN ip (192.168.20.2) got updated with my defined subdomin 51sec.fortiddns.com in the Intenet. On this configuration page, you also got a warning message, "the interface has a private ip address (192.168.20.2) which may not be publicly accessible".
In this example, the domain fortiddns.com is used. This domain is owned by Fortinet, as are the float-zone.comdomains and fortidyndns.com.