The prime factorization of 256 is $2^8$.
The proper divisors of 256 are 1, 2, 4, 8, 16, 32, 64, and 128.
Their sum is $1+2+4+8+16+32+64+128 = \boxed{255}$.
The answer is: 255
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
Determine the sum of all the proper divisors of 256.

### Answer:
