In this scenario, two branch offices each have a FortiGate unit and are connected in a gateway-to-gateway VPN configuration. One FortiGate unit has a domain name (example.com) with a dynamic IP address. See branch_2 in Figure 261.

Whenever the branch_2 unit connects to the Internet (and possibly also at predefined intervals set by the ISP), the ISP may assign a different IP address to the FortiGate unit. The unit has its domain name registered with a dynamic DNS service. The branch_2 unit checks in with the DDNS server on a regular basis, and that server provides the DNS information for the domain name, updating the IP address from time to time. Remote peers have to locate the branch_2 FortiGate unit through a DNS lookup each time to ensure the address they get is current and correct.

When a remote peer (such as the branch_1 FortiGate unit in Figure 261) initiates a connection to example.com, the local DNS server looks up and returns the IP address that matches the domain name example.com. The remote peer uses the retrieved IP address to establish a VPN connection with the branch_2 FortiGate unit.