The following specification is referenced in UPnP Device Architecture Version 1.0 and is provided here because it is no longer available from its original source.
UUIDs and GUIDs
This specification defines the format of UUIDs (Universally Unique IDentifier), also known as GUIDs (Globally Unique IDentifier). A UUID is 128 bits long, and if generated according to the one of the mechanisms in this document, is either guaranteed to be different from all other UUIDs/GUIDs generated until 3400 A.D. or extremely likely to be different (depending on the mechanism chosen). UUIDs were originally used in the Network Computing System and later in the Open Software Foundation's Distributed Computing Environment. (February 1997)