Subnetting Tools can be Wrong!

I just wanted to make a quick post on a conversation I couldn’t help but jump in on in the Network Engineering Professionals group on LinkedIn. Truthfully I ignore a lot of errors – and there are a lot of errors and incorrect statements/drawings/diagrams etc. that I simply do not understand how they make it into the feed. Most of the time others speak up and I simply chalk it up to a learning process for younger or newer networking folks.

That said, when I see a clear issue, and then dozens “like” or support or make comments like thank you to something that is wrong, my fingers hover over the keyboard as I anxiously try to hold back from getting involved. In this case I could not hold back.

So what is the issue? Someone posted that this tool called “pycidr” was great:

The thing that stood out to me was the class was wrong! Plus I was actually able to do this properly by hand faster that the installation. The error is a simple one, the tool is using the mask to try to figure out the class instead of the leading bits. I actually reported the issue to the tool author (there were no other bug reports!), so maybe this will get fixed….who knows.

Anyway, I had it installed and tested it myself, this time with a little supernetting:

Again, the class is wrong for the reason stated above. Plus, as far as I can tell the tool does not do IPv6.

So since I was updating my Wireshark profiles on github, I went looking and found this one:

The installation is simple:

sudo apt-add-repository -sy ppa:dreibh/ppa
sudo apt-get update
sudo apt-get install subnetcalc

Much Better!

Plus it does IPv6:

The takeaway? Learn to do it by hand for IPv4 (see my video here: ) and IPv6. Then be wary of tools that make errors.

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