Subnetting – “I Feel the Need, the Need for Speed”

Tom Cruise is wicked fast at subnetting!

Subnetting is King:

What is one of the most critical areas for students in the CCENT and CCNA exams when it comes to a pass or fail? Well, it is clear. Their ability to subnet fast and furious. And of course, accurately. This post reviews the approach I take in the exam. I am betting you have an even quicker method for speed subnetting – be sure to share it with all of us in the comments section below.

Be Careful:

Make sure you fully understand subnetting from a “long hand” binary perspective as well as the need for subnetting before you start quickly jumping to shortcut approaches like we do here. If you do not take the time to do this, you can have huge gaps in your understating that could cause severe career harm!

ICND1 Sample Question: 

Question: What is the last usable address in the subnet of a host with the address 192.168.1.133 and the subnet mask of 255.255.255.240?

Step 1: Since the is the first subnetting question I have encountered in my exam, I am going to use this as my opportunity to build my Powers of Two reference chart on my scratch paper.

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: How many bits of subnetting are used in the fourth octet here of our subnet mask (240)? My Power of Two chart tells me. 1 bit = 128; 2 bits = 192; 3 bits = 224; 4 bits = 240.

Step 3: I go four bits deep (from left to right) in the Power of Two chart. This tells me that the subnets increment on 16:

192.168.1.16…32…48…64…80…96…112…128…144

Step 4: We can see that this host lives on the 192.168.1.128 subnet. The broadcast address for this subnet is one less than the next subnet of 144, so that is 143. The last usable is 142. Our answer – 192.168.1.142.

What is great when you are practicing is that you can check your work and your understating! Visit the online subnet calculator to make sure you were right! Hey Cisco – give us online access in the exam 😉

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