Top Questions of the Week #13

Question

Now we're into post 13 of the "Top Question" of Super User in the Super User blog. Please post and vote for your favorite question for this week.

Please post any question that you feel is of worth and the reason why. Try not to promote your own questions or answers for publicity sake. We are looking for questions that are of similar par to those selected in the Super User Contest. If you like a posted question then vote it up. Each week we are going to try to post about the question and it's contents.

Answer 1

This QotW is now posted:
Windows 7 Network Awareness: How Windows knows it has an internet connection

This question from @nhinkle has sparked quite the interest given that it has only so few views:

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

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.

And the answers are quite interesting as well!