How to make a distributed node.js application?

Creating a node.js application is simple enough.

var app = require('express')();
app.get('/',function(req,res){
    res.send("Hello world!");
});

But suppose people became obsessed with your Hello World! application and exhausted your resources. How could this example be scaled up on practice? I don't understand it, because yes, you could open several node.js instance in different computers - but when someone access http://your_site.com/ it aims directly that specific machine, that specific port, that specific node process. So how?