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“A Globally-Unique Identifier (GUID) is generated algorithmically, usually using your network card’s MAC address (which should be unique) as well as the number of 100 nanosecond periods elapsed since 12:00 AM on October 15, 1582 (naturally).  Using these two pieces of data ensures that your computer won’t generate the same GUID twice and two different computers won’t generate the same GUID at the same time.  It’s not a “100% guarantee” but it’s very, very reliable.  What you get is something that looks like this (as a string):” source...