Many people have trouble adding new interfaces to Fortinet firewalls because they fail with adding the correct static routes and the correct metrics for those routes therefore fail at having users browse the internet succesfully.
Today I will document the process so that it isn't too dificult to decipher. The scenario is that the ISP puts down their router and gives you their IP addresses that we should use to configure the router, In our case we can assume that the ISP has given us 41.162.64.10/28.
If you aren't very good at subnet calculations then that isn't an issue for now, you can use the link https://www.calculator.net/ip-subnet-calculator.html add the ip address and the correct subnet and hit calculate and you would get the below results:
It is safe to assume that the IP 41.162.64.1 is the router IP address given by the ISP, ISP's generally take the first IP to configure the interface of the router which is then referred to as the default gateway on the firewall and we will use this later.
Let's start with the interface configuration:
Then we need to configure the static route and we done.
I will cover the adding of policies in the next blog which allows the user internet access.
Thanks for reading and happy firewalling.