Socket.io client doesn't connect to server

I have developed a simple chat using socket.io on a vagrant environment and it works correctly.

When I try to run the chat on the production environment, the nodejs/socket.io server runs but the client doesn't even fire the connect event. The only thing I've modified is, in the client side:

var socket = io.connect('http://localhost:3000');

to:

var socket = io.connect('http://ip_address:3000');

This is the server code:

var app = require('http').createServer(handler);
var io = require('socket.io')(app);
app.listen(3000, function() {
    console.log('Server is running');
});
function handler(req, res) {
    res.writeHead(200);
    res.end('');
}

io.on('connection', function(socket) {
    socket.on('subscribe', function() {
        console.log('subscribe request has arrived');
        console.log(socket.id);
    });
});