Using multiple firewalls (also known as Double NATing) can cause problems when attempting to get IPCOP working correctly. While Double NATing is discouraged, some situations prevent the removal of the other firewall.
To get IPCOP working correctly, you must make sure that you do not have a subnet overlap with any of your interfaces (Red, Orange, Blue, and/or Green).
Many firewalls use the following subnets:
It is recommended that you use subnets which are unlikely to be used. Below are suggested subnets which are unlikely to overlap with other routers/wifi hotspots (which may become an issue such as the tutorial given in section 3.1.2):