511

511 = 7 x 73.

511 is 111111111 in base 2 (binary). It is 777 in base 8.

511 is a Mersenne number.

511 is the smallest composite number that is the exact average of three consecutive primes (503, 509, and 521): (503 + 509 + 521)/3 = 511.

511 is a number that cannot be written as a sum of three squares.

The telephone number 511 is often used for traffic and travel information.

Source: Number Gossip

255

255 = 3 x 5 x 17.

255 = 11111111 in base 2.

255 is the largest value representable as an 8-bit binary number. Eight bits, otherwise known as a byte, is a very common size for storing "small" values in computer memories. Consequently, 255 appears as a bound or an otherwise magic number in many computer-related tasks. For example, 255 is the maximum amount of red, green, or blue that goes into an RGB color, as well as the maximum content of any of the four octets of an IP address.

255 is the least odd integer greater than 3 that is not the sum of two distinct powers of two and a prime.

255 is a number that cannot be written as a sum of three squares.

255 is 3333 in base 4.

255 is a Mersenne number: 255 = 2⁸ - 1.


Source: Number Gossip