Here are some related info:
http://forum.mikrotik.com/t/point-to-point-31-addresses/29492/1
particularly:
http://forum.mikrotik.com/t/point-to-point-31-addresses/29492/1
And this post is the one that (for me) is easier to remember, the concept of a mirror of a /32:
http://forum.mikrotik.com/t/point-to-point-31-addresses/29492/1