I know little about networking the Internet, but, from what I understand, it works — very approximately — as follows:

• I, sitting at the computer example.com, send a message saying, roughly, "get http://s.tk" to my ISP, which passes the message along, eventually to the machine at s.tk.
• The s.tk machine gets "example.comhas sent 'gethttp://s.tk'", so sendssomefileto its ISP which passes the file along, eventually to the machine atexample.com`.

When the file gets back to example.com, my computer, how does my computer know what to do with it? I'm sure the headers (or something else) indicate it's a Web page rather than, say, a Usenet post — that's not my question. My question is: how does it know whether to display the Web page in my open Opera window or my open Firefox window, or my other open Firefox window, or, heck, to open a new browser instance?