The octet is a unit of digital information in

computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, e ...

and

telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

that consists of eight

bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...

s. The term is often used when the term

byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...

might be ambiguous, as the byte has historically been used for storage units of a variety of sizes. The term ''octad(e)'' for eight bits is no longer common.

Definition

The international standard IEC 60027-2, chapter 3.8.2, states that a byte is an octet of bits. However, the unit

has historically been

platform Platform may refer to: Technology * Computing platform, a framework on which applications may be run * Platform game, a genre of video games * Car platform, a set of components shared by several vehicle models * Weapons platform, a system or ...

-dependent and has represented various storage sizes in the history of computing. Due to the influence of several major

computer architecture In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, t ...

s and product lines, the byte became overwhelmingly associated with eight bits. This meaning of ''byte'' is codified in such standards as ISO/IEC 80000-13. While ''byte'' and ''octet'' are often used synonymously, those working with certain

legacy system In computing, a legacy system is an old method, technology, computer system, or application program, "of, relating to, or being a previous or outdated computer system", yet still in use. Often referencing a system as "legacy" means that it paved ...

s are careful to avoid ambiguity. Octets can be represented using number systems of varying bases such as the

hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, hexa ...

decimal The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numeral ...

, or

octal The octal numeral system, or oct for short, is the radix, base-8 number system, and uses the Numerical digit, digits 0 to 7. This is to say that 10octal represents eight and 100octal represents sixty-four. However, English, like most languages, ...

number system A number is a mathematical object used to count, measure, and label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with number words. More universally, individual numbers can ...

s. The binary value of all eight bits set (or activated) is , equal to the hexadecimal value , the decimal value , and the octal value . One octet can be used to represent decimal values ranging from 0 to 255. The term ''octet'' (symbol: o) is often used when the use of ''byte'' might be ambiguous. It is frequently used in the

Request for Comments A Request for Comments (RFC) is a publication in a series from the principal technical development and standards-setting bodies for the Internet, most prominently the Internet Engineering Task Force (IETF). An RFC is authored by individuals or g ...

(RFC) publications of the

Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and a ...

to describe storage sizes of network protocol parameters. The earliest example is from 1974. In 2000,

Bob Bemer Robert William Bemer (February 8, 1920 – June 22, 2004) was a computer scientist best known for his work at IBM during the late 1950s and early 1960s. Early life and education Born in Sault Ste. Marie, Michigan, Bemer graduated from Cranbro ...

claimed to have earlier proposed the usage of the term octet for "8-bit bytes" when he headed software operations for

Cie. Bull Bull SAS (also known as Groupe Bull, Bull Information Systems, or simply Bull) is a French computer company headquartered in Les Clayes-sous-Bois, in the western suburbs of Paris. The company has also been known at various times as Bull General El ...

in France in 1965 to 1966. In

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

, French Canada and Romania, ''octet'' is used in common language instead of ''byte'' when the eight-bit sense is required; for example, a megabyte (MB) is termed a megaoctet (Mo). A variable-length sequence of octets, as in Abstract Syntax Notation One (ASN.1), is referred to as an octet string.

Octad

Historically, in Western Europe, the term ''octad'' (or ''octade'') was used to specifically denote eight bits, a usage no longer common. Early examples of usage exist in British, Dutch and German sources of the 1960s and 1970s, and throughout the documentation of Philips mainframe computers. Similar terms are ''triad'' for a grouping of three bits and ''decade'' for ten bits.

Unit multiples

Unit multiples of the octet may be formed with

SI prefix The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...

es and binary prefixes (power of 2 prefixes) as standardized by the

International Electrotechnical Commission The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...

in 1998.

Use in Internet Protocol addresses

The octet is used in representations of Internet Protocol computer network addresses. An

IPv4 Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version de ...

address consists of four octets, usually displayed individually as a series of decimal values ranging from 0 to 255, each separated by a full stop (dot). Using octets with all eight bits set, the representation of the highest-numbered IPv4 address is . An IPv6 address consists of sixteen octets, displayed in hexadecimal representation (two hexits per octet), using a colon character (:) after each pair of octets (16 bits are also known as hextet) for readability, such as .

Notes

References

External links

* {{DEFAULTSORT:Octet (Computing) Units of information