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, ...
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 tha ...
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 represented a ...
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
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 ...
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 ...
-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, the ...
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 systems 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, h ...
,
decimal, 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 systems. 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 ...
(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 ...
to describe storage sizes of network protocol parameters. The earliest example is from 1974. In 2000,
Bob Bemer claimed to have earlier proposed the usage of the term octet for "8-bit bytes" when he headed software operations for
Cie. Bull 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 regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans. Its metropolitan ar ...
,
French Canada and
Romania
Romania ( ; ro, România ) is a country located at the crossroads of Central, Eastern, and Southeastern Europe. It borders Bulgaria to the south, Ukraine to the north, Hungary to the west, Serbia to the southwest, Moldova to the east, a ...
, ''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
Abstract Syntax Notation One (ASN.1) is a standard interface description language for defining data structures that can be Serialization, serialized and deserialized in a cross-platform way. It is broadly used in telecommunications and computer n ...
(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
Koninklijke Philips N.V. (), commonly shortened to Philips, is a Dutch multinational conglomerate corporation that was founded in Eindhoven in 1891. Since 1997, it has been mostly headquartered in Amsterdam, though the Benelux headquarters is ...
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. ...
es and
binary prefix
A binary prefix is a unit prefix for multiples of Units of measurement, units. It is most often used in data processing, data transmission, and digital information, principally in association with the bit and the byte, to indicate multiplicatio ...
es (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 ...
in 1998.
Use in Internet Protocol addresses
The octet is used in representations of
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
computer network
A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections ar ...
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 d ...
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
An Internet Protocol Version 6 address (IPv6 address) is a numeric label that is used to identify and locate a network interface of a computer or a network node participating in a computer network using IPv6. IP addresses are included in the p ...
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 .
See also
*
Variable-width encoding
A variable-width encoding is a type of character encoding scheme in which codes of differing lengths are used to encode a character set (a repertoire of symbols) for representation, usually in a computer. Most common variable-width encodings a ...
Notes
References
External links
*
{{DEFAULTSORT:Octet (Computing)
Units of information