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A universally unique identifier (UUID) is an identifier standard used in software construction. A UUID is simply a 128-bit value. The meaning of each bit is defined by any of several variants. For human-readable display, many systems use a canonical format using hexadecimal text with inserted hyphen characters. For example: : de305d54-75b4-431b-adb2-eb6b9e546014The intent of UUIDs is to enable distributed systems to uniquely identify information without significant central coordination. In this context the word unique should be taken to mean "practically unique" rather than "guaranteed unique". Since the identifiers have a finite size, it is possible for two differing items to share the same identifier. This is a form of hash collision. The identifier size and generation process need to be selected so as to make this sufficiently improbable in practice. Anyone can create a UUID and use it to identify something with reasonable confidence that the same identifier will never be unintentionally created by anyone to identify something else. Information labeled with UUIDs can therefore be later combined into a single database without needing to resolve identifier (ID) conflicts. Adoption of UUIDs is widespread with many computing platforms providing support for generating UUIDs and for parsing/generating their textual representation. ==Definition== A UUID is a 16-octet (128-bit) number. In its canonical form, a UUID is represented by 32 lowercase hexadecimal digits, displayed in five groups separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters (32 alphanumeric characters and four hyphens). For example:: 123e4567-e89b-12d3-a456-426655440000The first 3 sequences are interpreted as complete hexadecimal numbers, while the final 2 as a plain sequence of bytes. The byte order is ''"most significant byte first (known as network byte order)"''(sec. 4.1.2) (note that GUID's byte order is different). This form is defined in the RFC〔(sec. 3) and simply reflects UUID's division into fields,〔(sec. 4.1.2) which apparently originates from the structure of the initial time and MAC-based version. The number of possible UUIDs is 1632, which is 2128 or about 3.4 × 1038. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Universally unique identifier」の詳細全文を読む スポンサード リンク
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