I'm trying to understand a cryptanalysis of the Blowfish cipher, and I need to calculate the probability of collision in the cipher's S-boxes. Basically an S-box is a list of 256 semi-random 32-bit numbers. We can assume that the values are random for convenience. I need to know how I calculate the probability that
$$ S(a) = S(a') $$$$ a \neq a' $$
where a and a' are positions in the S-box.