Octet (computing)
 | This articleneeds additional citations forverification.(September 2008) |
| octet | |
|---|---|
| Unit system | units derived from bit |
| Unit of | digital information, data size |
| Symbol | o |
| Inprimary units of information | 1 o = 8bits |
Theoctetis aunit of digital informationincomputingandtelecommunicationsthat consists of eightbits.The term is often used when the termbytemight be ambiguous, as the byte has historically been used for storage units of a variety of sizes.
The termoctad(e)for eight bits is no longer common.[1][2]
Definition
[edit]The international standardIEC60027-2, chapter 3.8.2, states that a byte is an octet of bits. However, the unitbytehas historically beenplatform-dependent and has represented various storage sizes in thehistory of computing.Due to the influence of several majorcomputer architecturesand product lines, the byte became overwhelmingly associated with eight bits. This meaning ofbyteis codified in such standards asISO/IEC 80000-13.Whilebyteandoctetare often used synonymously, those working with certainlegacy systemsare careful to avoid ambiguity.[citation needed]
Octets can be represented using number systems of varying bases such as thehexadecimal,decimal,oroctalnumber systems.The binary value of all eight bits set (or activated) is111111112,equal to the hexadecimal valueFF16,the decimal value25510,and the octal value3778.One octet can be used to represent decimal values ranging from 0 to 255.
The termoctet(symbol: o[nb 1]) is often used when the use ofbytemight be ambiguous. It is frequently used in theRequest for Comments(RFC) publications of theInternet Engineering Task Forceto describe storage sizes ofnetwork protocolparameters. The earliest example isRFC635from 1974. In 2000,Bob Bemerclaimed to have earlier proposed the usage of the term octet for "8-bit bytes" when he headed software operations forCie. Bullin France in 1965 to 1966.[3]
InFrance,French CanadaandRomania,octetis used in common language instead ofbytewhen the eight-bit sense is required; for example, a megabyte (MB) is termed a megaoctet (Mo).
A variable-length sequence of octets, as inAbstract Syntax Notation One(ASN.1), is referred to as an octet string.
Octad
[edit]Historically, inWestern Europe,the termoctad(oroctade) was used to specifically denote eight bits,[2][1]a usage no longer common. Early examples of usage exist in British,[2]Dutch and German sources of the 1960s and 1970s, and throughout the documentation ofPhilipsmainframe computers.[1]Similar terms aretriadfor a grouping of three bits anddecadefor ten bits.
Unit multiples
[edit]Unit multiples of the octet may be formed withSI prefixesandbinary prefixes(power of 2 prefixes) as standardized by theInternational Electrotechnical Commissionin 1998.
| SI prefixes | |||
|---|---|---|---|
| 1kilooctet(ko) | = 103octets | =1000octets | |
| 1megaoctet(Mo) | = 106octets | = 1000 ko | =1000000octets |
| 1gigaoctet(Go) | = 109octets | = 1000 Mo | =1000000000octets |
| 1teraoctet(To) | = 1012octets | = 1000 Go | =1000000000000octets |
| 1petaoctet(Po) | = 1015octets | = 1000 To | =1000000000000000octets |
| 1exaoctet(Eo) | = 1018octets | = 1000 Po | =1000000000000000000octets |
| 1zettaoctet(Zo) | = 1021octets | = 1000 Eo | =1000000000000000000000octets |
| 1yottaoctet(Yo) | = 1024octets | = 1000 Zo | =1000000000000000000000000octets |
| Binary prefixes | |||
|---|---|---|---|
| 1kibioctet(Kio, also written Ko, as distinct from ko) | = 210octets | =1024octets | |
| 1mebioctet(Mio) | = 220octets | = 1024 Kio | =1048576octets |
| 1gibioctet(Gio) | = 230octets | = 1024 Mio | =1073741824octets |
| 1tebioctet(Tio) | = 240octets | = 1024 Gio | =1099511627776octets |
| 1pebioctet(Pio) | = 250octets | = 1024 Tio | =1125899906842624octets |
| 1exbioctet(Eio) | = 260octets | = 1024 Pio | =1152921504606846976octets |
| 1zebioctet(Zio) | = 270octets | = 1024 Eio | =1180591620717411303424octets |
| 1yobioctet(Yio) | = 280octets | = 1024 Zio | =1208925819614629174706176octets |
Use in Internet Protocol addresses
[edit]The octet is used in representations ofInternet Protocolcomputer networkaddresses.[4] AnIPv4address 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 is255.255.255.255.
AnIPv6 addressconsists of sixteen octets, displayed in hexadecimal representation (twohexitsper octet), using a colon character (:) after each pair of octets (16 bits are also known ashextet) for readability, such as2001:0db8:0000:0000:0123:4567:89ab:cdef.[5]
See also
[edit]Notes
[edit]- ^However, theIEC 80000-13symbol "o" for octets can be confused with the postfix "o" to indicateoctal numbersinIntel convention.
References
[edit]- ^abc"Philips - Philips Data Systems' product range - April 1971"(PDF).Philips.1971. Archived fromthe original(PDF)on 2016-03-04.Retrieved2016-10-03.
- ^abcWilliams, R. H. (1969-01-01).British Commercial Computer Digest: Pergamon Computer Data Series.Pergamon Press.ISBN1483122107.978-1483122106.
- ^Bemer, Robert William(2000-08-08)."Why is a byte 8 bits? Or is it?".Computer History Vignettes.Archived fromthe originalon 2017-04-03.Retrieved2017-05-15.
[…] I came to work forIBM,and saw all the confusion caused by the 64-character limitation. Especially when we started to think about word processing, which would require both upper and lower case. […] I even made a proposal (in view ofSTRETCH,the very first computer I know of with an 8-bit byte) that would extend the number ofpunch cardcharacter codes to 256 […]. So some folks started thinking about 7-bit characters, but this was ridiculous. With IBM's STRETCH computer as background, handling 64-character words divisible into groups of 8 (I designed the character set for it, under the guidance of Dr.Werner Buchholz,the man who DID coin the term "byte"for an 8-bit grouping). […] It seemed reasonable to make a universal 8-bit character set, handling up to 256. In those days my mantra was" powers of 2 are magic ". And so the group I headed developed and justified such a proposal […] TheIBM 360used 8-bit characters, although not ASCII directly. Thus Buchholz's "byte" caught on everywhere. I myself did not like the name for many reasons. The design had 8 bits moving around in parallel. But then came a new IBM part, with 9 bits for self-checking, both inside the CPU and in thetape drives.I exposed this 9-bit byte to the press in 1973. But long before that, when I headed software operations forCie. Bullin France in 1965-66, I insisted that "byte" be deprecated in favor of "octet". […]
- ^Kozierok, Charles M. (2005-09-20) [2001]."The TCP/IP Guide - Binary Information and Representation: Bits, Bytes, Nibbles, Octets and Characters - Byte versus Octet".3.0.Archivedfrom the original on 2017-04-03.Retrieved2017-04-03.
- ^R. Hinden;S. Deering(February 2006).IP Version 6 Addressing Architecture.Network Working Group.doi:10.17487/RFC4291.RFC4291.Draft Standard.ObsoletesRFC3513.Updated byRFC5952,6052,7136,7346,7371and8064.
External links
[edit]
The dictionary definition ofoctetat Wiktionary
