How to find ip address of remove device when working with Modbus

I need to develop a server. This server is required for the binding of "smart home" with the phone (mobile application). "Smart home" controller connects to the server and sends id. Server must remember "key"-"value" where "key" id id of devide and "value" - ip address of controller. How to determine the ip address of the remote device. To implement this i can use PHP or NodeJs.

I shall be grateful for any help