The docs assume two systems on the network, one with host running FreeRTOS system in qemu and another with the echo server, but you should be able to run both on same machine. The demo bridges the qemu ethernet with the host’s ethernet, so you will have so set the FreeRTOS mac address to a unique value for your network. The FreeRTOS IP should be an unused local IP that doesn’t conflict with a device on your network. The subnet mask should match your network’s . The echo server’s address is the IP of the machine you are running the echo server on.
Two systems on the network could also be (tested) lets say a Windows, MacOS, or Ubuntu hosts (running the echo server), with a VM running Ubuntu guest and qemu running FreeRTOS on them, so you do not need two physical machines. Just one Machine with a VM running on it. You could possibly run them all on a Linux machine without a VM.
The scenario (with the VM) would be:
FreeRTOS running with Qemu from within Ubuntu (guest) would send a packet to the echo server running on Windows, MacOS or Linux (host), and message would be displayed on the echo server, showing that FreeRTOS TCP stack was able to send the message through.