255 (number)

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255 (two hundred [and] fifty-five, CCLV) is the natural number following 254 and preceding 256.

Cardinal Two hundred [and] fifty-five
Ordinal 255th
Factorization [255 = 3 \cdot 5 \cdot 17]
Roman numeral CCLV
Binary 11111111
Hexadecimal FF

In mathematics

Its factorization makes it a sphenic number. Since 255 = 2⁸ - 1, it is a Mersenne number, and the fourth such number not to be a prime number.

In base 10, it is a self number.

255 is a repdigit in base 2 (11111111) in base 4 (3333), and in base 16 (FF)

In computing

255 is a special number in some tasks having to do with computing. This is the maximum value representable by an eight-digit binary number, and therefore the maximum representable by an unsigned 8-bit byte (the most common size of byte, also called octet), the smallest common variable size used in high level programming languages (bit being smaller, but rarely used for value storage). The range is 0 to 255, which is 256 total values.

[255 = 2^8 - 1 = \mbox_ = 11111111_2.]

For example, 255 is the maximum value

that can be assigned to elements in the 24-bit RGB color model, since each color channel is allotted eight bits.
of any dotted quad in an IP address.
of the Alpha blending scale in Delphi (255 being 100% visible and 0 being fully transparent)

This number occurs especially frequently in video games when a small number is needed:

There are exactly 255 levels in Pac-Man.
In The Legend of Zelda, the maximum number of rupees you may have at one time is 255.
In Super Mario Bros., the maximum amount of lives you can possess is exactly 255. The game does not check if this number is exceeded; getting another life makes all of them disappear.
In Warcraft II, the maximum mana for any magic unit is 255.
In EarthBound, the maximum character level and character stat value is 255.
In Pokémon, although the limit on levels is 100, you can exceed it with a cheat code. Passing level 255 results in the number resetting to 0; however, your stats are not reset.
In some Final Fantasy games, the maximum value of most stats is 255.
In Sword of Hope, the maximum gold you can carry is 255.
In StarCraft, the maximum number of kills a unit can obtain is 255, and after using a map editor, the maximum upgrades to attack/armor/shields is 255.
In Dragon Warrior Monsters and its sequel the highest amount of the Agility stat your monsters can have is 255. Also its Wild rating can reach 255 as well.

The usage of 8 bits for storage in older videogames has had the consequence of it appearing as a hard limit in many videogames. It was often used for numbers where casual gameplay would not cause anyone to exceed the number. However in most situations it is reachable given enough time. This can cause many other peculiarities similar to the above listed to appear when the number wraps back to 0.

This number could be interpreted by a computer as -1 if a programmer is not careful about which 8-bit values are signed and unsigned, and the two's complement representation of -1 in a signed byte is equal to that of 255 in an unsigned byte. The maximum value of a signed byte is 127.

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Cardinal	Two hundred [and] fifty-five
Ordinal	255th
Factorization	[255 = 3 \cdot 5 \cdot 17]
Roman numeral	CCLV
Binary	11111111
Hexadecimal	FF