The architecture of our industrial control systems is not as static as it was some years ago. The adapting of new standards, or simply trying to improve the security of our industrial networks, creates the need to introduce one or various firewalls within our network. Thinking about having to change a network's architecture, the IP of our devices, tests, etc. when introducing a new firewall often leads to the bad decision to not install it. But, do we know about transparent firewalls and how they can be installed with almost no impact in our network? These solutions have advanced a lot in the industry and may be a true plus to our security.