Routing through the firewall
Is it possible to setup your computer to be a firewall for other computers that are on the same network with just one Ethernet card instead of two cards. From what I tried before, I had to route the packets from one card, that is connected to WAN, to a second ethernet card, and the second Ethernet card is then connected to other computers. Also, I don't want the ip addresses to be different from the source. I appreciate your help!
What is the point of having a firewall if all networks are in the same layer2 network? You could do it with 1 interface when using vlans but that is virtually 2 seperate networks again.
You can shut down NAT by enabling advanced outbound nat at firewall>nat, outbound tab. Delete all nat mappings that it creates for you in the table at the bottom.