web button script to broadcast UDP message

I have created a single HTML web page with 13 buttons in JPG format. I want these image buttons, when pressed, to broadcast a UDP message (ASCII) to computers on the same subnet. E.g. button 1 will send the text "ONE" to it subnet, etc. No response is expected from the UDP server and the HTML page always look static. I already have a UDP server. To keep things simple no binary message will be used. The challenge is, the page will be hosted on a public server, e.g. [login to view URL] but the buttons will broadcast to the local subnet only, e.g. broadcast to computers on [login to view URL] and of course the UDP server will run on the same local subnet listening to any UDP client, only the web page is hosted externally. Preferred web server technology is html/javascript/php, will consider asp too. Source code and parameters (subnet, message content, etc) must be configurable / editable by me. The code must be compatible with any modern PC and mobile devices (android, iphone, tablets etc) running readily available internet browsers. No custom plug-in or modules to be installed on client devices. Any question feel free to ask.