express node.js

How to bind http server with express object to a specific ip address?

How do i bind a specific address to a http object? Currently the default ip address for node.js is and I want to change it to

var express = require('express');
var app = express();
var http = require('http').Server(app);
http.listen(3000, "");

How to achieve that?

edited: I added but the default is still