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The Windows 7 network icon down in the system tray (sorry, taskbar notification area) shows your connectivity status - but more than just whether the connection is on or off, it also apparently can tell if your Internet connection is down.
Case in point: my Internet connection went down, and the icon changed to this:
Note the little yellow exclamation point overlay. Windows put this icon there just after my Internet connection went down - less than a minute I would say, although I didn't time it.
It's worth mentioning that my local LAN was still up and running fine.
My question is... how did Windows know this?
Is it monitoring network traffic and watching for failures? Did the router (a modern uPnP router) somehow tell Windows that the WAN connection had gone down? Does Windows periodically ping certain servers to test Internet connectivity?
(p.s. I mention Windows 7 because that's what I have and I don't recall if Vista had the same behavior as I didn't have it long enough to find out. I do remember that Windows XP and earlier did not have this "feature.")