Modbus Coms on LocalHost

I can't wrap my head around this for some reason...

Are you able to establish Modbus communications on the same hardware.

i.e. Beckhoff IPC communicating via Modbus to Ignition Software installed on the IPC.

Yes, you can have a ModbusTCP master and slave communicate between each other that are on the same PC. I test AdvancedHMI ModbusTCP driver (client) by running a ModbusTCP simulator (server) on the same computer. You can either reference it by localhost, 127.0.0.1, or by the current IP address of the computer. Even though the slave and master have the same IP address, this works because they have different TCP/IP ports and the IP stack routes the packets based on the ports.

Yes. I have a lengthy post about Modbus TCP on here that you can search for, but the long and short is that it uses three pieces of information to send a message to/from server (slave) and client (master): IP address, Port, and Modbus Node Number. Each IP address has tens of thousands of ports and each port can host 240 node numbers, so you could have thousands of clients and servers running on the same IP address (including localhost)