Transparent Mode
Transparent mode is so named because the device is effectively transparent in that it does not appear on the network in the way that other network devices show as a nodes in the path of network traffic. Transparent mode is typically used to apply the FortiOS features such as Security Profiles etc. on a private network where the FortiGate unit will be behind an existing firewall or router.
These are some of the characteristics of Transparent mode:
• The FortiGate unit is invisible to the network.
• All of its interfaces are on the same subnet and share the same IP address.
• The FortiGate unit uses a Management IP address for the purposes of Administration.
• Still able to use NAT to a degree, but the configuration is less straightforward
In Transparent mode, you can also perform NAT by creating a security policy or policies that translates the source addresses of packets passing through the FortiGate unit as well as virtual IP addresses and/or IP pools.