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I've read that Patch Tuesday is the second Tuesday of every month, so I could picture the code in Windows Update on the client machine being set to contact the company servers so Microsoft doesn't have to find all the Windows computers- all the Windows computers come to them on that day.

But if there's a critical update on some random day like the third Wednesday of the month so critical that everyone really needs it as soon as possible, how does Microsoft initiate a connection to each applicable Windows computer connected to the Internet? Most people have non-static IP addresses, I'd suspect, and I don't think most internet service providers are keeping databases of customer computers including what OS the computer they're connecting to the Internet is running.

So how does Microsoft find and initiate contact with Windows Update on all Windows computers if it needs to?

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What you describe sounds like a dark, dystopian future. – William Jackson Jan 13 '12 at 20:31
up vote 27 down vote accepted

Microsoft doesn't have to do that – the computers themselves have Windows Update running and check for available updates every day.

The Patch Tuesday is just a convention, for users to be able to install all accumulated updates at once, and system administrators to prepare.

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