Expected Hamming distance of a set of binary strings sampled without replacement

I initially have all length-$L$ binary strings. Suppose I randomly sample without replacement $k\le 2^L$ strings from this set. What is the expected pair-wise hamming distances of the sampled $k$ binary strings?

• How is the Hamming distance of $k$ strings defined? I only know the definition for two strings. – joriki Mar 7 '16 at 12:45
• A pair of string has a hamming distance, what is the expectation of all pairs. – Adam I. Mar 7 '16 at 13:00

The method of sampling is irrelevant here; the individual pairs are uniformly sampled from all pairs, so the expected Hamming distance is just the expected Hamming distance of a uniformly sampled pair, which is $L/2$ (or, in case you were referring to the sum over all pairs, $\binom k2\cdot\frac L2$).