Cheng's Blog

The HTTP 1.1 specification states:

A single-user client SHOULD NOT maintain more than 2 connections with any server or proxy.

But many modern browsers supports more than that:

IE 6 and 7:      2
IE 8:            6
IE 9:            6
IE 10:           8
IE 11:           8
Firefox 2:       2
Firefox 3:       6
Firefox 4 to 17: 6
Opera 9.63:      4
Opera 10:        8
Opera 11 and 12: 6
Chrome 1 and 2:  6
Chrome 3:        4
Chrome 4 to 23:  6
Safari 3 and 4:  4

The connection limitation is per hostname not IP address. [1]

Thus, even a.test.com and b.test.com points to the same ip address, you can still double the number of parrallel connections.

source: http://stackoverflow.com/questions/985431/max-parallel-http-connections-in-a-browser