Octet (computing)

# Octet (computing)

Discussion

Encyclopedia
An octet is a unit of digital information
Units of information
In computing and telecommunications, a unit of information is the capacity of some standard data storage system or communication channel, used to measure the capacities of other systems and channels...

in computing
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...

and telecommunications that consists of eight bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

s. The term is often used when the term byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

might be ambiguous, as there is no standard for the size of the byte.

## Overview

The unit byte is not standardized and has represented various storage sizes in the history of computing. However, due to the influence of several major computer architectures and product lines, the byte became overwhelmingly associated with 8 bits. While to most people today, byte and octet are synonymous, those working with certain legacy system
Legacy system
A legacy system is an old method, technology, computer system, or application program that continues to be used, typically because it still functions for the users' needs, even though newer technology or more efficient methods of performing a task are now available...

s are careful to avoid ambiguity.

Octets are often expressed and displayed using a variety of representations, for example in the hexadecimal
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...

, decimal
Decimal
The decimal numeral system has ten as its base. It is the numerical base most widely used by modern civilizations....

, or octal
Octal
The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. Numerals can be made from binary numerals by grouping consecutive binary digits into groups of three...

number system
Number system
In mathematics, a 'number system' is a set of numbers, , together with one or more operations, such as addition or multiplication....

s. The binary value of all 8 bits set (or turned on) is 11111111, equal to the hexadecimal value FF, the decimal value 255, and the octal value 377.

Octets are used in the representation of Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...

computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....

IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...

address consist of four octets, usually shown 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 255.255.255.255.

A variable-length sequence of octets, as in Abstract Syntax Notation One
Abstract Syntax Notation One
Data generated at various sources of observation need to be transmitted to one or more locations that process it to generate useful results. For example, voluminous signal data collected by a radio telescope from outer space. The system recording the data and the system processing it later may be...

(ASN.1),
is referred to as an octet string.

The international standard IEC 60027-2, chapter 3.8.2, says that a byte is an octet of bits.

## Origin

The word octet comes from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

and Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

numerical prefix
Numerical prefix
Number prefixes are prefixes derived from numbers or numerals. In English and other European languages, they are used to coin numerous series of words, such as unicycle – bicycle – tricycle, dyad – triad – decade, biped – quadruped, September – October – November – December, decimal – hexadecimal,...

octo, meaning eight.

## Use

The term octet is often used when the use of byte might be ambiguous, as there is no standard for the size of the byte. It is frequently used in the Request for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...

(RFC) publications of the Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...

to describe storage sizes of network protocol parameters. The earliest example is RFC 635 from 1974.

In France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

and Romania
Romania
Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...

, octet is used in common language instead of byte when the 8-bit sense is required, for example, a megabyte (MB) is called a megaoctet (Mo).

## Unit multiples

Octets can be used with SI prefix
SI prefix
The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...

es or the binary prefix
Binary prefix
In computing, a binary prefix is a specifier or mnemonic that is prepended to the units of digital information, the bit and the byte, to indicate multiplication by a power of 2...

es (power of 2 prefixes) as standardized by the International Electrotechnical Commission
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...

in 1998.
 1 kibioctet (Kio) = 210 octets = 1024 octets 1 mebioctet (Mio) = 220 octets = 1024 Kio = octets 1 gibioctet (Gio) = 230 octets = 1024 Mio = octets 1 tebioctet (Tio) = 240 octets = 1024 Gio = octets 1 pebioctet (Pio) = 250 octets = 1024 Tio = octets 1 exbioctet (Eio) = 260 octets = 1024 Pio = octets 1 zebioctet (Zio) = 270 octets = 1024 Eio = octets 1 yobioctet (Yio) = 280 octets = 1024 Zio = octets

The SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

prefixes kilo, mega, giga, tera, etc., stay the same as for all the SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

units, based on power of 10.
In this case:
 1 kilooctet (ko) = 103 octets = 1000 octets 1 megaoctet (Mo) = 106 octets = 1000 ko = octets 1 gigaoctet (Go) = 109 octets = 1000 Mo = octets 1 teraoctet (To) = 1012 octets = 1000 Go = octets 1 petaoctet (Po) = 1015 octets = 1000 To = octets 1 exaoctet (Eo) = 1018 octets = 1000 Po = octets 1 zettaoctet (Zo) = 1021 octets = 1000 Eo = octets 1 yottaoctet (Yo) = 1024 octets = 1000 Zo = octets