Subnetting – What Mask to Use

SPeed Subnetting


Remember, it is totally worth repeating in this series of posts on subnetting – while practicing just how fast you can ace subnetting questions in the certification environment is always worthwhile and fun, you do want to make sure you understand exactly why you are going through the exercise to being with. You also want to be sure you understand how subnetting fundamentally works. This is often referred to as subnetting “longhand”.

ICND1 Sample Subnetting Question:

Your co-worker has decided upon use of the address space for a section of your network. This section requires 14 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 formula for the number of subnets you can create based on subnet bits is 2 ^ S where S is the number of subnet bits borrowed. From the chart I see if we “borrow” 4 bits we can create 16 subnets. This is two more than we need in this scenario, but that is great as it will allow for future expansion with ease.

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


3 thoughts on “Subnetting – What Mask to Use

  1. You might want to add an additional row underneath the powers of 2 you already have to show how the subnetmask would correlate with each power of two.

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