From Sega Retro
In mathematics, hexadecimal or simply hex is a numeral system with a radix or base of 16 usually written using the symbols 0–9 and A–F or a–f.
For example, the decimal numeral 79 whose binary representation is 01001111 can be written as 4F in hexadecimal (4 = 0100, F = 1111).
It is a useful system in computers because there is an easy mapping from four binary numeral bits to a single hex digit. A byte can be represented as two consecutive hexadecimal digits. However, this mixed representation (Arabic digits up to nine and for zero, other symbols for the six other hexadecimal digits) is ambiguous and needs prefix, suffix or subscripts to be clear.
There are many ways to denote hexadecimal numerals:
There is no single agreed-upon standard, so all the above conventions are in use, sometimes even in the same paper. However, as they are quite unambiguous, little difficulty arises from this.
The most commonly used (or encountered) notations are the ones with a prefix "0x" or a subscript-base 16, for hex numbers. For example, both 0x2BAD and 2BAD16 represent the decimal number 11181 (or 1118110).
The word "hexadecimal" is strange in that hexa is derived from the Greek language|Greek έξι (hexi) for "six" and decimal is derived from the Latin for "ten". An older term was the pure Latin "sexidecimal", but that was changed because some people thought it too risqué, and it also had an alternative meaning of "base 60". However, the word "sexagesimal" (base-60) retains the prefix.
A common use of hexadecimal numerals is found in HTML and CSS. They use hexadecimal notation to specify colors on web pages; there is just the # symbol, not a separate symbol for "hexadecimal". Twenty-four-bit color is represented in the format #RRGGBB, where RR specifies the value of the Red component of the color, GG the Green component and BB the Blue component. For example, a shade of red that is 238,9,63 in decimal is coded as #EE093F. This syntax is borrowed from the X Window System. See Web colors.
In URLs, special characters can be coded hexadecimally, with a percent sign used to introduce each byte; e.g., http://en.wikipedia.org/wiki/Main%20Page
The canonical written form of numeric IPv6 addresses represents each group of 16 bits as a separate hexadecimal number, to ease reading and transcription of the 128-bit addresses.
As with other numeral systems, the hexadecimal system can be used in forming fractions, although recurring decimals are common:
Because the radix 16 is a square (42), hexadecimal fractions have an odd period much more often than decimal ones. Recurring decimals occur when the denominator in lowest terms has a prime factor not found in the radix. In the case of hexadecimal numbers, all fractions with denominators that are not a power of two will result in a recurring decimal.
See also: SCHG:Number Systems