I purchased Todd's new 6th Edition CCNA Study Guide for the 640-802 Exam. I just finished chapter three and need help understanding Private Reserved Address Spaces.

I'd like to know if these address spaces can be calculated or if they're supposed to just be remembered?

I think I understand why the Class A Reserved Address Space is - and why Class C is - However, the Class B Reserved Address Space of doesn't make sense to me.

I've searched online for a method to calculate this hoping to understand how/why/where the 172.31 comes from but haven't had any success.

I'd greatly appreciate any help in understanding this. If I can understand it, itll help me understand how to arrive at the correct answer for #8 in the Written Lab 3.1: TCP/IP Section of the End of Chapter questions.