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- 八位元組
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In computing, an octet is a grouping of eight bits.
Octet, with the only exception noted below, always refers to an entity having exactly eight bits. As such, it is often used where the term byte might be ambiguous. For that reason, computer networking standards almost exclusively use octet. It is prominently used in Request for Comments (RFCs) published by the Internet Engineering Task Force. The earliest example is RFC 635 from 1974. In France, French Canada and Romania, the word octet is commonly used instead of byte (in the 8-bit sense); a megabyte (MB) is called a megaoctet (Mo).
Exception
The term "permission octet" is sometimes used for the group of basic permission bits (read, write, execute) of many file systems for Unix-like platforms. As these three bits can represent 8 different values octal notation is also used. The correct term "permission triplet" is also common though.
Normalization
Octets can be used with SI prefixes or the power of 2 prefixes as standardized by the International Electrotechnical Commission in 1998.
- kibi for « kilo binary » ;
- mebi for « mega binary » ;
- gibi for « giga binary » ;
- tebi for « tera binary » ;
| 1 kibioctet (Kio) | = 210 octets | = 1,024 octets | |
| 1 mebioctet (Mio) | = 220 octets | = 1,024 Kio | = 1,048,576 octets |
| 1 gibioctet (Gio) | = 230 octets | = 1,024 Mio | = 1,073,741,824 octets |
| 1 tébioctet (Tio) | = 240 octets | = 1,024 Gio | = 1,099,511,627,776 octets |
| 1 pebioctet (Pio) | = 250 octets | = 1,024 Tio | = 1,125,899,906,842,624 octets |
| 1 exbioctet (Eio) | = 260 octets | = 1,024 Pio | = 1,152,921,504,606,846,976 octets |
| 1 zebioctet (Zio) | = 270 octets | = 1,024 Eio | = 1,180,591,620,717,411,303,424 octets |
| 1 yobioctet (Yio) | = 280 octets | = 1,024 Zio | = 1,208,925,819,614,629,174,706,176 octets |
The SI prefixes kilo, mega, giga, tera, etc., stay the same as for all the SI units, based on power of 10. In this case:
| 1 kilooctet (ko) | = 103 octets | = 1,000 octets | |
| 1 megaoctet (Mo) | = 106 octets | = 1,000 ko | = 1,000,000 octets |
| 1 gigaoctet (Go) | = 109 octets | = 1,000 Mo | = 1,000,000,000 octets |
| 1 teraoctet (To) | = 1012 octets | = 1,000 Go | = 1,000,000,000,000 octets |
| 1 petaoctet (Po) | = 1015 octets | = 1,000 To | = 1,000,000,000,000,000 octets |
Origin
The word octet (meaning "group of eight") comes from the Latin and Greek numerical prefix octo, meaning eight.
