When working with Modbus/TCP there is sometimes a need to tap the network traffic. On Ethernet, this is more difficult in comparison to serial networks (RS232/RS485) due to the switched architecture of Ethernet. But it is not impossible, various techniques exist to help you here, each with its own advantages and disadvantages.
I have written a non-commercial article about it to help you determine the best way to do this. For the PDF, please visit: