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A globally unique identifier (GUID) is a 128-bit number created by the Windows operating system or another Windows application to uniquely identify specific components, hardware, software, files, user accounts, database entries and other items.
GUIDs are part of the universally unique ID (UUID) standard that is used in Windows and Windows applications.
GUIDs were initially developed to keep track of instances of Component Object Model (COM) objects and are still used to identify COM DLLs in the Windows registry.
GUIDs were created with an algorithm that employed the user's media access control (MAC) address. This system was later dropped because users were concerned that documents could be traced back to individual machines. GUIDs can now be created in a number of different ways using a combination of unique settings.