When studying TCP/IP networking (specifically v4), I can't even begin to count how often I had to answer questions like

    Which of the following IP addresses are on the 10.3.103.128/28 subnet:

1. 10.3.103.127
2. 10.3.103.131
3. 10.3.104.244
4. 10.3.103.144

I have attached two versions of the cheat sheet that I used to get through some of the most annoying practice exercises.

Fortunately, because of the way the data is laid out, if you use it enough, you won't need to refer to the sheet anymore.  Also, because its structure clearly shows the patterns that show up in powers of two, you can quickly reproduce the necessary portions during certification exams, so you can spend time thinking about the real problems, and not getting stuck in bitwise arithmetic.

The first file is a one page version of the sheet, meant to be printed out and kept in a notebook.

The second file is a two page version.  It is means to be printed out on two sides of a CD label sized piece of paper.  It can easily be kept in an emergency CD kit that many admins keep on hand.


Referring to the example above, and using the 2 page version of the cheat sheet:

First, notice that we're dealin with a /28 nework.

Looking at the legend on the bottom of the sheet, we see that a /28 covers half of a single column.
And since /28 is in the fourth column, it represents the fourth octet of an IPv4 address.

So, we flip to the back side of the cheat sheet, and see the top half of the first column (in yellow) represents all the octets that would fit...  10.3.103.128-10.3.103.143.

So, working through the examples is very simple

1. No.  Outside the range, so not same subnet.
2. Yes.  Inside range, same subnet.
3. No.  Since we're looking at fourth octet, the first three must match, and they don't.
4. No.  Just outside the range.
   

Need to find out if 10.4.100.4 and 10.4.101.33 are on the same /21?
Looking at the legend, we see that the third octets must live within the same "quarter column", in this case the non-bolded numbers that are in the top of the last column on the front side.
10.4.97.xx-10.4.112.xx are on the same /21, so the answer is yes.


As a bonus, I've built in a Hex and Binary table as well as a subnet mask reference.


Feel free to enjoy.



Single page cheat sheet

Two page CD-Sized cheat sheet


Usual copyright stuff applies... that is, if you're using this for yourself, go ahead.  If you you want to include it with a collection, distribute copies to your department, duplicate it and charge for it, etc.  please contact me first at copier@wonkified.net.