Returning rand() % N does not uniformly give a number in the range [0, N), unless N divides the length of the interval into which rand() returns (i.e. is a power of 2).

How to do it correctly

unsigned int rand_interval(unsigned int min, unsigned int max)
    int r;
    const unsigned int range = 1 + max - min;
    const unsigned int buckets = RAND_MAX / range;
    const unsigned int limit = buckets * range;
    /* Create equal size buckets all in a row, then fire randomly towards
     * the buckets until you land in one of them. All buckets are equally
     * likely. If you land off the end of the line of buckets, try again. */
        r = rand();
    } while (r >= limit);
    return min + (r / buckets);


To change the range of rand() is to divide it into boxes; for example, if RAND_MAX == 11 and you want a range of 1..6, you should assign {0,1} to 1, {2,3} to 2, and so on. These are disjoint, equally-sized intervals and thus are uniformly and independently distributed.