Comment by buddydvd

+1 for skip32.c since it's O(1) space and O(1) time. With the prime approach, you may need to call it multiple times to reject numbers greater than 2^32-1. Also, a generalized Feistel algorithm may be used to generate permutations with fewer than 32 bits with the same constant space/time requirement and may be helpful in skipping specific IP ranges.