In this blog, I describe how to create an overload nat pool on a FortiGate which has in basic a point-to-point internet connection and a separate subnet for other services. In this particular situation, I had a /30 subnet for the point-to-point connection. Which i configured on the WAN1 interface. The free address range is a /29.
Create a new interface which isn’t in use on your fortigate.
Create in the firewall rules the necessary rules to have normal internet access. Except 1 in rule you need to change a setting.
In the Nat section you select instead of Use Outgoing Interface Address you choose Use Dynamic IP Pool. You can create this IP pool from this section of you create one via the menu IP Pools
Choose a logical name for the IP pool. And select 1 IP address from the free available to use IP Addresses. Copy past this <address>–<copy address>, Save the settings and choose withing the policy towards the Internet access the IP Pool address which you want to use.
After configuring this settings test if everything works. You don’t neet to add an IP route in the static routes. As the subnet is already known by the ISP it will forward al the traffic towards that IP address when you start using it.