In this example procedure, you will add a FortiGate unit to an existing network without changing the configuration of the network. The network is connected to the Internet using a router that performs NAT. Without replacing the existing router, the FortiGate unit will block access from the Internet to the private network but allow users on the private network to connect to the Internet.
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The original article/video can be found at Inserting a FortiGate unit without changing the network configuration (Transparent mode)