Do the other devices run a program of yours? You can broadcast UDP messages to all connected devices. They can respond with their ip address. I haven't done it myself however I remember seeing a discussion about it in the forum.
The device is a sales receipt printer, with an USB port.
I want to use a Wi-Fi key connected to the USB port and send a string (with my Android App) to the device via Wi-Fi.
I do not know if this will be possible.