You want to put your WiFi users on the same network segment (or subnet) as your wired LAN users, but the FortiGate unit requires each network interface to have a single unique network segment. The solution is to create a software switch interface with the internal LAN interface and WiFi network virtual interfaces as members.

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The original article/video can be found at FortiGate Cookbook – WiFi and Wired Clients on the Same Subnet