Here’s how it works:
This little chart applies to any subnet mask, simply take the meaningful octet (the octet that is not 255 or 0) and apply what you need to know, such as:
A host has an IP address of 192.168.111.42/27 (/27 being CIDR notation – used literally in the syntax of Cisco NX-OS CLI & universally important to know, see here for more info).
We know that it has a sub-net mask of 27 1’s, shown in binary as:
11111111.11111111.11111111.11100000
(11111111=128+64+32+16+8+4+2+1=255), so (11100000=128+64+32=224) or, .224… and specifically 255.255.255.224.
So from a manually calculated CIDR notation of /27 we have a sub-net mask of 255.255.255.224. If you are studying to be a network engineer one should just strait up memorize each slash notations /1 – /32 corresponding sub-net mask, but don’t forget where it came from.
So, we have our meaningful octet from our sub-net mask, .224, and from that we can gather from the chart that the mask dictates that the class C sub-net is to be broken down into 8 separate blocks who’s network addresses span 32 IPs. **Remember this does not mean that you have 32 IPs available for hosts to use, the 32nd IP address is the second subnet’s network address, the 64th IP address is the third, and so on. The last IP address in a block is always reserved for that sub-nets broadcast address, and you loose one to the next block’s network address. So that’s useful, but how are the numbers tied together?
Well, the top line is just powers of 2 starting with 2. Easy enough
The second line shows the value of the broken down binary octet (8 bits per octet with a 1(power on) meaning network bit and a 0(power off) being a host bit. All 1’s or 0’s are consecutive in sub-net masks, you will never see a 10110101 or anything like it.)
If you had a mask of .224 like above, you have 11100000 (128+64+32) as shown in the chart. The CIDR notation /27 already has 24 single 1’s behind the 3 in the meaningful octet, so you just take 24 & add 3 instead of counting all 27.
The third line shows the actual sub-net mask. It is calculated from adding the block sizes of all columns leading up to the meaningful octet of the sub-net mask.
In the .224 example, this was calculated by adding 128+64+32=224.
So, to write this chart out you don’t need to memorize it. Just know:
1. Powers of 2 starting from 2. (3rd grade math)
2. Binary values of 8 bit numbers used in IP.4 addressing. (Should know this long before you get to sub-net calculations.)
3. Simple addition. (ask a kinder-gardener if you need help)
I made up this little charge about 4 years ago in preparation for the first Cisco exam I ever took. While it’s very useful to jot down before a test begins on your scratch paper, if you use it in conjunction with your multiples of 16, 32, 64(most common sub-net sizes that require thinking) charts enough during study like I did, you’ll find that you don’t even need the scratch pad.
Memorizing this little chart & being able to recite your multiples tables (see below) is all one needs to do IPv.4 subnet calculations in your head.
David P. Roberts