Thursday, February 28, 2019

Another Subnet Calculator!!! Why???








Yes I finally took the time to create one using Google Sheets.  During one of my interviews I was asked a series of subnet questions and I thought “why not just create a subnet calculator and point them to the calculator that I had created”. So that’s what I did.

You can find the link that asks you to copy the Subnet Calculator on the Links page.  This gives you your own personal version to work with.

This subnet calculator requires some usage explanation to avoid incorrect results.  Otherwise it is pretty easy to use.

The first thing that I have noticed is that when it loads initially it shows the results as “Loading…” or “Name…” in the results section.  See an example of this behavior below.
This makes you think that it is broken.  So don’t panic, just use the 4th oct drop down on the Netmask row and select a value. This will re-initialize the sheet to display correct values.

It is a free form subnet calculator, so you can mess yourself up if you are not paying attention.  By the way, it does have a conditional validation popup, but it only works if the field is selected and you hover over the selector.

Here is the rule to follow.

Rule: You can select any value of the left octet netmask if the right netmask octet is equal to 0.

Here are some example pictures:

Correct

Incorrect


As you can see the 4th octet is 255. It should be 0 as the octet to the left is less than 255. As you can see this does not work as there should be more than 1 host available. I have some ideas I am toying around with on fixing this calculator and may do so in the future. Future articles will explain how subnetting works as well as how I built the calculator and what functions I had to create to make it work.

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