Point-to-point IPv4 links are typically addressed with a 30-bit mask ( This only provides 50% addressing efficiency (only two of the four addresses per subnet are usable), but many organizations consider this sufficient. Consequently, an engineer often needs to find the address of the interface at the other end of a /30 link to a neighboring router.

You could do the binary in your head to deduce whether the local interface has the lower or higher IP, but an easier way is to remember this rule of thumb: the lower address will always be odd, and the higher will always be even.