I have a problem that I hope is easy for you guys to help me with.

At work we have 4 computers that currently are not connected to the internet, only to a local network for our point of sale system. Our debit machines are connected to the PoS computers by RJ45 to serial cables and the debit terminals are connected through an unmanaged switch to our modem to access the internet. Our debit processing company is forcing us to change terminals and these new ones take ethernet in and send it to the computers to communicate through TCP/IP instead of serial. That will force the PoS computers to have access to the internet. We would rather they didn’t have access to the net. I called our ISP today to see if MAC address filtering was a possibility on our modem and it is not. So I am looking for a simple solution to keep the NIC of each of the computers functional but prevent them from accessing the internet.

Would simply upgrading the switch they are on to a managed one or a router, be all I need to do so I can set up a MAC filter? If so any suggestions on one would be appreciated. Is it even possible for wired connections? If not any help would be greatly appreciated. Thanks in advance!