Communicate with node.js script from weppage to machine

I'm pretty new to node.js so my explanation may be a little hard to understand:S

So I'm working on a solution for a problem i have. I have a machine where i got a Node.js javascript code looping. It's controlling a robot that should be able to recieve commands.

Then i also got my webpage(running on a normal host). How can i for example send a instruction to my Node.js script running on my robot(different machine).

If several people where on the webpage and pressing the buttons(robot control), i want the Node.js script be able to handle the different instructions it recieves without crashing.

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Explanation in stages: User visit webpage->press button on weppage->webpage script sends direction command->robots computer listening and receives the command->robot runs the correct function depending on which command it recieved