Stve post # 14 has the answer.
If You donīt have a gateway defined, the PC will not be able to access the internet.
Just leave it blank.
Also, you can be on many subnets at the same time.
Your CNC machine can be on say 10.10.1.0 - network address, and your PCs on another.
If your primary IP address is on the non-internet connected one, even the more clever programs wont be able to use it.
/cp/network/properties/add ip address
e.g. 192.168.1.254 as a secondary ip to your plasma PC.
It will then see network shares, and can share files, but wont be able to access the internet.
The solution is perfectly safe.
Some rare mac-based stuff like auto-finding printers/pokeys autoconfig/csmio-ip-s autoconfig did not work, in the past, with 2 ip addresses.
I donīt know if they have fixed these bugs.