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 2: You have run the ipconfig command and discovered your IP address and mask are 192.168.20.102 and 255.255.255.224. How many hosts are permitted on your subnet?
Step 1: I reference the Powers of Two chart I created on my scratch paper when I encountered the first question. Adding 128 + 64 + 32 = 224. There are 3 bits used for subnetting and that leaves 5 bits for hosts.
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: The equation for the number of hosts per subnet is 2^h – 2 where h is the number of host bits. From the chart I see that 2^5 = 32. 32-2 = 30 hosts per subnet! Too easy!
As always, let us know in the comments if you have a quicker approach.
Source :: http://blog.ine.com/2010/10/14/speed-subnetting-part-2/