Octet (computing)
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In computing, an octet is a grouping of eight bits.
Octet refers to an entity having exactly eight bits. As such, it is often used where the term byte might be ambiguous. For that reason, computer networking standards almost exclusively use octet. It is prominently used in Requests for Comments (RFCs) published by the Internet Engineering Task Force. The earliest example is RFC 635 from 1974. In France, French Canada and Romania, the word octet is commonly used instead of byte (in the 8-bit sense); a megabyte (MB) is called a megaoctet (Mo).
Early in its history, in the age of computer science preceding tabletop and desktop microcomputers (the earliest of which almost all used 8-bit data words,) the word byte was not standardized to 8 bits and could indicate a unit of different sizes (all relatively small) in different contexts and for different machines. Hence, while to most people today, byte and octet are synonyms, those working with certain legacy systems need to be careful to avoid ambiguity.
Although octets are a series of eight binary digits, they are often formulated or displayed as hexadecimal, decimal, or octal values. The binary value of all 8 bits set (or turned on) is 11111111; equal to the hexadecimal value of FF, the decimal value of 255, and the octal value of 377.
Octets are used in Internet Protocol (IPv4) computer network addresses. These consist of a series of four octets, usually shown in decimal and separated by dots. Using octets with all eight bits set, the largest representation of a network address (although an invalid address) would be 255.255.255.255.
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[edit] Normalization
Octets can be used with SI prefixes or the power of 2 prefixes as standardized by the International Electrotechnical Commission in 1998.
- kibi for « kilo binary » ;
- mebi for « mega binary » ;
- gibi for « giga binary » ;
- tebi for « tera binary » ;
| 1 kibioctet (Kio) | = 210 octets | = 1,024 octets | |
| 1 mebioctet (Mio) | = 220 octets | = 1,024 Kio | = 1,048,576 octets |
| 1 gibioctet (Gio) | = 230 octets | = 1,024 Mio | = 1,073,741,824 octets |
| 1 tebioctet (Tio) | = 240 octets | = 1,024 Gio | = 1,099,511,627,776 octets |
| 1 pebioctet (Pio) | = 250 octets | = 1,024 Tio | = 1,125,899,906,842,624 octets |
| 1 exbioctet (Eio) | = 260 octets | = 1,024 Pio | = 1,152,921,504,606,846,976 octets |
| 1 zebioctet (Zio) | = 270 octets | = 1,024 Eio | = 1,180,591,620,717,411,303,424 octets |
| 1 yobioctet (Yio) | = 280 octets | = 1,024 Zio | = 1,208,925,819,614,629,174,706,176 octets |
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:
| 1 kilooctet (ko) | = 103 octets | = 1,000 octets | |
| 1 megaoctet (Mo) | = 106 octets | = 1,000 ko | = 1,000,000 octets |
| 1 gigaoctet (Go) | = 109 octets | = 1,000 Mo | = 1,000,000,000 octets |
| 1 teraoctet (To) | = 1012 octets | = 1,000 Go | = 1,000,000,000,000 octets |
| 1 petaoctet (Po) | = 1015 octets | = 1,000 To | = 1,000,000,000,000,000 octets |
[edit] Origin
The word octet (meaning "group of eight") comes from the Latin and Greek numerical prefix octo, meaning eight.
[edit] Misuse of the term
In documentation of file systems for Unix-like operating systems[1], a confused term, "permission octet",[2] was sometimes used instead of the correct permission triplet for the group of 3 bits used as permission bits (read, write, execute). The fact that 3 bits can represent 8 different values and that octal notation is often used may perhaps explain this misunderstanding. The term octal digit should be used instead.
[edit] References
- ^ Dhanjani, Nitesh (2003). Linux and UNIX Security Portable Reference. Hack Notes. McGraw-Hill Professional. pp. 146. ISBN 0072227869. http://books.google.com/books?id=Ly_C9AhR8K0C&pg=RA1-PA146&lpg=RA1-PA146&source=web&ots=CROzTt-MFC&sig=_Gp0C-5_7AkvAVtb5OCVLxx-ly0#PRA1-PA148,M1. Retrieved 2009-01-28.
- ^ "System Administration Guide: Security Services". docs.sun.com. Sun Microsystems, Inc.. 2008. http://docs.sun.com/app/docs/doc/816-4557/6maosrjhd?a=view. Retrieved 2009-01-28.
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