@frits : This is my first GW, but during my early experiments I run it as an Ethernet GW, then I reconfigured to MQTT.
Currently I don't have a controller running but I'm using Node-Red + Influx-DB + Grafana.
So probably during these first experiments, while in Ethernet mode, the Node_IDs were distributed.
I will erase the EEPROMs by the next opportunity and will check the behaviour of my Nano sensors after re-boot.
Thanks for your explanation frits, it helped me to learn more about MySensors.