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. 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 with all 8 bits set (or turned on) of 11111111 is equal to a hexadecimal value of FF, a decimal value of 255, and an 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.

[edit] 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.

[edit] 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 » ;

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:

[edit] Origin

The word octet (meaning "group of eight") comes from the Latin and Greek numerical prefix octo, meaning eight.