So I must disclaim that I know that this is not an idea solution, however wanted to get some science and technical terms correct for running two networks off one NIC card as a work around to having VLAN and/or second router. Again, this is NOT how I would do things, but for some home user wanting to run lets say a network off of another network for testing that it can be done this way because of how layer 2 works. The layer 2 part is what I am most interested in, but wanted to get some expert opinions and why it works.
So a home user has a PC that is hooked into a dumb switch. The dumb switch then goes to a normal home router, however one port goes to a switch to a garage for instance. The home network is 192.168.1.0 and the garage is 10.10.10.0 that runs some devices that do not need internet. Just needs to be able to send and receive data, and those devices might have another device down the line like a switch with more devices. Now I know that you can set the NIC card on the computer to be the main IPv4 of 192.168.1.10 255.255.255.0 192.168.1.1 18.104.22.168 etc on the main page and on the advanced tab you can add a secondary IP of 10.10.10.10 255.255.255.0 no gateway and it will route the traffic to that network with zero issues and normal LAN traffic will go out the other direction.
I guess call it a proof of concept, but it allows this happen because it is only doing layer 2 and switched go off of MAC addresses so that traffic will never hit the router for this reason. Now just want to see if their is a bandwidth reduction or overhead to be aware of because of the split networks? See the visio diagram below.