Universally Unique Identifier (UUID) A universally unique identifier (UUID) is a 128-bit number that identifies unique Internet objects or data. A UUID is generated by an algorithm with values that are based on a machine's network address. UUIDs are used by many software companies, such as Microsoft and Apple, and are extensively used as components of Microsoft’s globally unique identifiers (GUIDs). Other UUID uses include Linux’s ext2/ext3 file system.