65535

65535, written with a comma as 65,535, is the integer that represents the largest value of an unsigned 16-bit integer. It equals 2^16 − 1 and has a binary representation of 16 consecutive ones (1111111111111111) and a hexadecimal representation of FFFF.

In computing, 0 through 65,535 are the values typically stored by unsigned 16-bit integers. The signed variant of 16-bit integers ranges from −32,768 to 32,767, so 0xFFFF corresponds to −1 in two’s complement representation. Systems that use 16-bit addressing have a 64-kilobyte address space, with addresses from 0 to 65,535.

65535 also appears in color and networking contexts. In some 16-bit color formats that pack color channels into 16 bits (for example, 5 bits red, 6 bits green, 5 bits blue, known as RGB565), the entire 16-bit word can range from 0x0000 to 0xFFFF, allowing 65,536 distinct color values. In networking, 65535 is the maximum port number for TCP and UDP, and the maximum size of a UDP datagram length field is 16 bits, so total UDP length cannot exceed 65,535 bytes (including header).

Mathematically, 65,535 factors as 3 × 5 × 17 × 257, the product of several Fermat primes. It is also 2^16 − 1, a Mersenne-like number that is not prime. Its properties arise from the factorization of 2^n − 1 for n = 16.