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