[math-fun] Golomb rulers

Hello, ... I am confused, having just read this on Eric Weisstein site: "However, the unique optimal Golomb 4-mark ruler is (0, 1, 4, 6), which measures the distances (1, 2, 3, 4, 5, 6) (and is therefore also a perfect ruler). As a further example, it turns out that the length of an optimal 6-mark Golomb ruler is 17. In fact, there are a total of four distinct 6-mark Golomb rulers, all of length 17, one of which is given by (0, 1, 4, 10, 12, 17). " http://mathworld.wolfram.com/GolombRuler.html ... What is the difference between a "perfect ruler" and an "optimal ruler" ?! ... I cannot measure 14 or 15 with (0, 1, 4, 10, 12, 17) Best, É.