I’m going to show you a little bit of how to use a software application called Wireshark — which was formerly known as Ethereal. It allows you turn your Ethernet/wireless adaptor on “listen” mode, and you can watch packets as they go to and from your machine. I’m not getting this into a huge networking discussion, but just realize that there are little bits of information called packets that go to and from machines as you do things on the Internet.
For our purposes, we will be concerned with the transfer of packets between two machines in particular, your machine, and the machine of the webserver where your favorite website is hosted.

» More: Wireshark Quick Tutorial (Useful for post PHP: HTTP Authentication)