Using an IP address parameter in the creation of a Socket.io socket

Question

In the following snippet, a tutorial author shows how to alter the original tutorial to include an http server. Here's the snippet.

var http = require(‘http’),  
fs = require(‘fs’),  
io = require(‘socket.io’),  
index;  
fs.readFile(‘./chat.html’, function (err, data) {  
 if (err) {
    throw err;
 }
 index = data;
});
var server = http.createServer(function(request, response) {  
  response.writeHeader(200, {“Content-Type”: “text/html”});
  response.write(index);
  response.end();
}).listen(1223);
//and replace var socket = io.listen(1223, "1.2.3.4"); with:
var socket = io.listen(server); 

The code in the original tutorial didn't include the http server, and socket was defined as simply:

var socket = io.listen(1223, "1.2.3.4");

I noticed that he replaces the variable's content io.listen(1223, "1.2.3.4"); with server which doesn't include the ip ( 1.2.3.4 ) anywhere.

My Question:

• What is the purpose/effect of the referenced IP address?
• Why is it excluded when passing an http server to create the socket?

Answer 1

When you are listening on a port, you can optionally include the IP address of a specific interface to listen on. For example, you might have several network interfaces with several IP addresses, and only want your service running on one of them. A more common use case is that you only want your server accessible on localhost, so you might have it listen only on 127.0.0.1 .

Now, when you call io.listen(server) where server is an existing Node.js HTTP server, Socket.IO isn't actually opening a new listening connection at all. This is a shortcut for Socket.IO to wrap its methods on the existing HTTP server. If you wanted to specify a specific interface address to listen on, you would need to do it where .listen() is called on the HTTP server, above where you call io.listen(server) .

More info in the documentation for raw network sockets in Node.js: http://nodejs.org/api/net.html#net_server_listen_port_host_backlog_callback