WARNING: You must master subnetting using our course or some other trusted materials before you start using these shortcut approaches. It is a common issue for Cisco candidates to move directly to subnetting shortcuts for the exams without fully understanding exactly how subnetting functions.
Question 3: Your co-worker has decided upon use of the 172.16.0.0 address space for a section of your network. This section requires 15 subnets. What subnet mask will you recommend?
Step 1: I reference the Powers of Two chart I created on my scratch paper when I encountered the first question. The forumla for the number of subnets you can create based on subnet bits is 2^s. From the chart I see if we “borrow” 4 bits we can create 16 subnets.
2^7=128 | 2^6=64 | 2^5=32 | 2^4=16 | 2^3=8 | 2^2-=4 | 2 ^1=2 | 2^0=1
Step 2: Borrowing 4 bits beyond the Class B boundary results in 255.255.128+64+32+16 = 240. Our mask is 255.255.240.0.
Source :: http://blog.ine.com/2010/10/17/speed-subnetting-part-3/