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Octet (computing)
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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 Requests for Comments (RFCs) published by the Internet Engineering Task Force. The earliest example is RFC 635 from 1974.
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Encyclopedia
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 Requests 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).
Early in its history, in the age of computer science preceding tabletop and desktop microcomputers (the earliest of which almost all used 8-bit data words,) the word byte was not standardized to 8 bits and could indicate a unit of different sizes (all relatively small) in different contexts and for different machines. Hence, while today to most people byte and octet are synonyms and byte is not ambiguous, historically it was not always so, and occasionally, especially in legacy system contexts, it still might be. This provides a justification of the use of the term octet for absolute specificity.
Although octets are a series of eight binary digits, they are often formulated or displayed as hexadecimal, decimal, or octal values. The binary value of all 8 bits set (or turned on) is 11111111; equal to the hexadecimal value of FF, the decimal value of 255, and the octal value of 377.
Octets are used in Internet Protocol (IPv4) computer network addresses. These consist of a series of four octets, usually shown in decimal and separated by dots. Using octets with all eight bits set, the largest representation of a network address (although an invalid address) would be 255.255.255.255.
Other uses
The term "permission octet" is rarely used to mean the group of basic permission bits (read, write, execute) of file systems for Unix-like operating systems. As these three bits can represent 8 different values, octal notation is often used. The more appropriate term "permission triplet" is also sometimes used.
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 | = octets | | 1 mebioctet (Mio) | = 220 octets | = Kio | = octets | | 1 gibioctet (Gio) | = 230 octets | = Mio | = octets | | 1 tebioctet (Tio) | = 240 octets | = Gio | = octets | | 1 pebioctet (Pio) | = 250 octets | = Tio | = octets | | 1 exbioctet (Eio) | = 260 octets | = Pio | = octets | | 1 zebioctet (Zio) | = 270 octets | = Eio | = octets | | 1 yobioctet (Yio) | = 280 octets | = Zio | = 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 | = octets | | 1 megaoctet (Mo) | = 106 octets | = ko | = octets | | 1 gigaoctet (Go) | = 109 octets | = Mo | = octets | | 1 teraoctet (To) | = 1012 octets | = Go | = octets | | 1 petaoctet (Po) | = 1015 octets | = To | = octets |
Origin
The word octet (meaning "group of eight") comes from the Latin and Greek numerical prefix octo, meaning eight.
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