Subnetting for the CompTIA Network+ N10-008 Exam

I wanted to share this subnetting tip.  I came from simiangeek  on the CompTIA sub-Reddit .  I am also including some links to other training resources that I have found

I wanted to share my experience, hope it will work for some other people. I went ahead and got to a point where I could jot down the class C CIDR table like this:

*I don’t think they’ll ask you about a /31, all the questions I got about point-to-point were using /30 but you should know that it could be used as point-to-point for some cases.

Now, looking at this chart is a bit overwhelming, right? But, there’s some really simple things going on here. First off, write down the first column, all the notation (/XX). Then the next column. Starts at 2 and doubles up to 128. Third column? Just the opposite. Start with 2 at the bottom and work your way back up. Third column is just the value of the second column minus two (for available addresses). Subnet mask column is the only one that really requires any math, but it’s still simple–take the value of the previous line’s mask, then you just add the value of the hosts column for the this line (/26 = 128 + 64 = 192).

So there are really only two things that you need to really *know*–A /25 network has 2 networks and is .128. Fill out the headers of your chart like above (Network, Hosts, Available, Mask) and the rest is simple fill-in-by-sequence. At this point, with this method, I can literally slap this chart down in under a minute on paper.

Here are the other resources.  First some video collections on subnetting

Practice Subnetting IPv4:

This is my favorite video series on subnetting.

Sunny Classroom – Subnetting Videos

Professor Messer’s Seven Second Subnetting

Subnetting Video

Network+ N10-007 Performance Based Questions (PBQ)  This PBQ was still on the N10-008 when I took the exam

CompTIA sub-Reddit

Reddit Net+ articles

Reddit articles by bobwyzguy

WyzGuys Reference Desk 

The refence desk is a collection I put together early in my career, and posted on my website as a quick and easy reference for myself.  This has turned out to be one of my most frequently visited pages.  There is an extensive  section on networking from when I was studying for the CCNA.

Wireless Access Point Configuration Simulators

One of your PBQs is going to involve configuring and access point.  Spend plenty of time getting familiar with the configuration interface.  The Linksys E1500 is similar to the GUI on the exam.



About the Author:

I am a cybersecurity and IT instructor, cybersecurity analyst, pen-tester, trainer, and speaker. I am an owner of the WyzCo Group Inc. In addition to consulting on security products and services, I also conduct security audits, compliance audits, vulnerability assessments and penetration tests. I also teach Cybersecurity Awareness Training classes. I work as an information technology and cybersecurity instructor for several training and certification organizations. I have worked in corporate, military, government, and workforce development training environments I am a frequent speaker at professional conferences such as the Minnesota Bloggers Conference, Secure360 Security Conference in 2016, 2017, 2018, 2019, the (ISC)2 World Congress 2016, and the ISSA International Conference 2017, and many local community organizations, including Chambers of Commerce, SCORE, and several school districts. I have been blogging on cybersecurity since 2006 at


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