For Part 2 of this series – click here.

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 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

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4 Responses to “Speed Subnetting, Part 3”

  1. Bob says:

    Even talking about classful networks should be a shooting offence, and every time you mention the word “Class B” you should add that these classes have been deprecated for like 15 years, and have no bearing on the real world besides the occational historic bug that shows up.

  2. Zeke says:

    @Bob. I’ve ceased even commenting about the term “Class X Netmask” (which is incorrect but used constantly). I’ve even caught myself saying it (how else does one describe it to management?). Don’t start complaining about correct usage!

  3. Great tips!
    I’ll link you your tips from my blog. I like them all for their simplicity!


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