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A globally unique identifier (GUID, or ) is a unique reference number used as an identifier in computer software. The term "GUID" typically refers to various implementations of the universally unique identifier (UUID) standard. GUIDs are usually stored as 128-bit values, and are commonly displayed as 32 hexadecimal digits with groups separated by hyphens, such as: : 21EC2020-3AEA-4069-A2DD-08002B30309DThey may or may not be generated from random (or pseudo-random) numbers. GUIDs generated from random numbers normally contain 6 fixed bits (these indicate that the GUID is random) and 122 random bits; the total number of unique such GUIDs is 2122 (approximately 5.3×1036). This number is so large that the probability of the same number being generated randomly twice is negligible; however other GUID versions have different uniqueness properties and probabilities, ranging from guaranteed uniqueness to likely duplicates. Assuming uniform probability for simplicity, the probability of one duplicate would be about 50% if every person on earth as of 2014 owned 600 million GUIDs. ==Common uses== * Microsoft Windows uses GUIDs internally to identify the classes and interfaces of COM objects. A script can activate a specific class or object without having to know the name or location of the dynamic linked library that contains it. Because of this, ActiveX, a system for downloading and installing controls in a web browser, uses GUIDs to uniquely identify each control. * Intel's GUID Partition Table, or GPT, a system for partitioning hard drives. * JT files use a partitioning into 4+2+2+8 *1 bytes to represent nodes in the data structure and segment IDs. * ''Second Life'' uses GUIDs for identification of all assets in its world. * Database developers and administrators may use GUIDs as primary keys for database tables to ensure uniqueness between database servers, at the cost of making the working set size for caching much larger for a relational database server, potentially impacting application performance. 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「Globally unique identifier」の詳細全文を読む スポンサード リンク
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