An octet is another name for byte. A byte has eight bits. This unit of digital information used to be common in computing and telecommunications when the term byte was ambiguous, since historically there was no standard definition for the size of the byte. The usage of the old term octad(e) for 8 bits is no longer common today.
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Overview
The unit byte is platform-dependent 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. This meaning of byte is codified in such standards as ISO/IEC 80000-13. While to most people presently, byte and octet are synonymous, those working with certain legacy systems are careful to avoid ambiguity.
Representation of octets
Octets are often expressed and displayed using a variety of representations, for example in the hexadecimal, decimal, or octal number systems. The binary value of all 8 bits set (or activated) is 7002255000000000000♠111111112, equal to the hexadecimal value 7002255000000000000♠FF16, the decimal value 7002255000000000000♠25510, and the octal value 7002255000000000000♠3778. One octet can be used to represent decimal values ranging from 0 to 255.
Octets in IPv4 and IPv6
Octets are used in the representation of Internet Protocol computer network addresses.
An IPv4 address consists 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.
An IPv6 address consists of sixteen octets, shown using hexadecimal representation (two digits per octet) and using a colon character (:) after each pair of octet for readability, like this FE80:0000:0000:0000:0123:4567:89AB:CDEF. If a pair or more consecutive octets equal zero it may be replaced by two following colon characters (::) but this can be used only once in a given IPv6 address to avoid ambiguity. The previously given IPv6 address can thus also be written as FE80::0123:4567:89AB:CDEF. In addition leading zeroes may also be omitted as they are not significant bits in the address. Applying this to the previous example mentioned will result in an IPv6 address of FE80::123:4567:89AB:CDEF.
Definitions
A variable-length sequence of octets, as in Abstract Syntax Notation One (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.
Use
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 (RFC) publications of the Internet Engineering Task Force to describe storage sizes of network protocol parameters. The earliest example is RFC 635 from 1974.
In France, French Canada and Romania, octet is used in common language instead of byte when the 8-bit sense is required, for example, a megabyte (MB) is termed a megaoctet (Mo).
Historically, the term octad (or octade) was used to specifically denote 8 bits as well at least in Western Europe; however, this usage is no longer common today. The exact origin of this term is unclear, but it can be found in British, Dutch and German sources of the 1960s and 1970s, and throughout the documentation of Philips mainframe computers. Similar terms exist in common English such as triad for a grouping of three and decade for ten.
Unit multiples
Octets can be used with SI prefixes or the binary prefixes (power of 2 prefixes) as standardized by the International Electrotechnical Commission during 1998.
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: