|You could use two SOHO Routers to accomplish this.|
You would daisychain the second to the first and set it up to have a completely diffferent (separate) subnet from the one connected to the internet.
Click on my name above and read my "how-to" guide titled, "◦Add a second Router to your LAN"
You'll want the scenario where you have two separate subnets and you connect the routers using the "Lan port to WAN port" connection.
Just FYI, there is no way to do what you want without spending some money. My solution is the least expensive.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.