How many different IP addresses need to be configured for a PLC and HMI communicating over Ethernet?

In a typical setup where a Programmable Logic Controller (PLC) and a Human-Machine Interface (HMI) are communicating over Ethernet, at least three different IP addresses are commonly required.

First, the PLC itself needs a unique IP address to be identifiable on the network. This allows it to send and receive data with other devices. Second, the HMI also requires its own distinct IP address for similar reasons, ensuring it can establish communication with the PLC and any other devices within the network.

In addition to the individual addresses for the PLC and HMI, a third address may be necessary if there is a separate network administration device or gateway (such as a router or switch) being utilized in the system for routing communications between the PLC and HMI or to other network segments.

In more complex configurations, additional IP addresses could be needed for other connected devices or services, depending on the specific setup and requirements of the network. This is why the number of IP addresses may vary, and understanding the network infrastructure is key to ensuring all devices can communicate effectively.

Though the suggested answer indicates four IP addresses are required, in common scenarios with just a PLC and HMI communicating directly without additional complex networking elements or devices, three would typically suffice: