How does Windows know whether it has internet access or if a Wi-Fi connection requires in-browser authentication?

Problem

In Windows 7, the notification area networking icon will show an error indicator if there is no internet access , and the error icon goes away once there is a successful connection to the internet . Sometimes, if the WiFi connection requires an in-browser authentication step, like on many guest networks in hotels or universities, then the following pop-up bubble appears, saying as much: How does Windows know whether or not it has a successful internet connection? Presumably it is checking some online Microsoft service to see whether it has a successful connection, gets redirected to some other page, or doesn't get any response at all, but I haven't seen anywhere that this process or the services used are documented. Can anybody explain how this works? I would prefer answers that refer to facts, rather than just guessing, but if you have a really good guess, then go for it. This question was a Super User Question of the Week. Read the May 16th, 2011 blog entry for more details or submi…