The octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as the byte has historically been used for storage units of a variety of sizes. The term octad for eight bits is no longer common.
Definition
A variable-length sequence of octets, as in Abstract Syntax Notation One, is referred to as an octet string. The international standard IEC 60027-2, chapter 3.8.2, states that a byte is an octet of bits. However, the unit byte has historically been platform-dependent and has represented various storage sizes in the history of computing. Due to the influence of several major computer architectures 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, decimal, or octal number systems. The binary value of all eight bits set 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 is often used when the use of byte might be ambiguous. It is frequently used in the Request for Comments publications of the Internet Engineering Task Force 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, French Canada and Romania, octet is used in common language instead of byte when the 8-bit sense is required, for example, a megabyte is termed a megaoctet.Historically, in Western Europe, the term octad was used to specifically denote 8 bits; however, this usage is no longer common. Early examples of usage exist in British, Dutch and German sources of the 1960s and 1970s, and throughout the documentation of Philipsmainframe computers. Similar terms are triad for a grouping of three and decade for ten.
The octet is used to represent Internet Protocolcomputer 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. 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 and using a colon character after each pair of octets for readability, like this FE80:0000:0000:0000:0123:4567:89AB:CDEF. If two or more consecutive octets equal zero they 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.
