The Art of the Subnet Cheat Sheet
And Other Subnetting Tips & Tricks
As I've grown older, I've grown less tolerant of memorizing useless information. After hearing people talk of memorizing a subnetting "cheat sheet," to help on Cisco exams, my thought was I would just memorize the formulas and do the math instead of memorizing yet more useless information. However, that mistake became painfully obvious as I failed the Cisco INTRO exam because I ran out of time before I was finished with the test. And because I scored over 700 points on what I did complete, I'm reasonably sure I would have passed if I had been able to complete all of the questions.
When I first started doing subnetting, I was actually doing boolean AND operations, and to figure out the proper subnet mask, I was writing out the netmask in binary and converting it to decimal. While I'm sure it is good to know all of that, Cisco let a nugget of truth slip out in its CCNA Self-Study ICND Exam Certification Guide (emphasis added):
Using the binary math required to find the subnet number and broadcast address really does help you understand subnetting to some degree. To get the correct answers faster on the exam, you might want to avoid all the conversion and binary math.
The following subnet cheat sheet will not solve all of the subnetting questions on the exam. What it will do, however, is gain valuable time. Since you can't take anything into the exam, the trick is to write the following chart out on your dry-erase board before you start the exam:
|Hosts||Netmask||Number of Subnets|
And if you examine the chart very closely, you can actually reproduce it with very little memorization. Here is the way to do it. First, duplicate column one of the table, which is fairly easy, and then fill in the second column, which is nothing but multiples of two, starting out at four. (if you aren't good with multiplication, when you get to the higher numbers, you can actually just write the numbers out to the side twice and add them together to get the number for the next row).
Next, fill in the netmask for the /24 network and the /16 network, which should also be easy to remember (if you are about to take the exam and can't remember /24 and /16 netmasks, you might as well hang it up). The netmask for the /30 network is also fairly easy to remember, but if you forget any of the netmasks, all that needs to be done is subtract the number of hosts directly to the left of it to get the next netmask. For example, 255.255.255.252 provides for four hosts (two useable, because zero is the network and .4 is the broadcast address). If you take 252 and subtract 4 from it, you get the netmask for the next row, 255.255.255.248. If you take 255.255.255.248 and subtract its 8 hosts, you get 255.255.255.240, which is the netmask for the next row. This works all the way down to the /24 network.
The only odd netmask to memorize is the /23 netmask, which is 255.255.254.0. That is also not hard to remember because it is just one off from the .255 directly above it. Note that after the /23 network, all of the network masks are identical to the /25 - /30 networks, just move them over one octet to the left. So from /22 to /17, you already have the needed information, just fill it in.
For the fourth column, just put a 1 in the /24 network and put in multiples of two up the chart from the /24 network, as well as down the chart, to the /16 row.
As you can see, once you understand the table, it can be reproduced with very little memorization.
How to use the chart
Now that you have the chart, if a simulation question calls for a /27 netmask, instead of writing it out in binary and converting it to decimal, you can just refer to the chart and plug in the netmask 255.255.255.224.
If the exam question asks for the network and broadcast address of a host, for example, 192.168.1.68 /27, simply look at the hosts provided by the /27 network mask, which is 32. Now simply add by 32 until you get to a subnet higher than the .68 host (be sure to add 32 each time and not get into using multiples of 2, which is easy to do here. If you get the numbers, 32, 64, 128, 256, you doing it wrong and will miss questions on the exam). What you should come up with is this:
Since the address is 192.168.1.68, it must fall in the subnet between 64 and 128. And since the first address of the subnet is the network address and the last address of the subnet is the broadcast, the network address is 192.168.1.64 and the broadcast address is 192.168.1.95.
If the exam asks to find a network that will allow for 4 subnets and at least 48 hosts per subnet, just look at the table and pick the row that matches:
Hosts Netmask Number of Subnets /26 64 255.255.255.192 4
If a question involves two IP addresses, for example 184.108.40.206 /28 and 220.127.116.11 /28 and the questions asks if they are in the same network, just look at the hosts provided by the /28 netmask, which is 16. Count by 16 until you pass the networks involved:
Since you know that the network address is 18.104.22.168 and the broadcast address is 22.214.171.124, then you know both IP addresses are in the same network.
The Cisco exams are geared toward people who are sharp at math. If you aren't a math wizard, you will be at a disadvantage to those who are, because a good portion of the INTRO and ICND exams deal with subnetting. If you aren't good with math, the subnetting cheat sheet will only help so much (and it won't help with converting binary or hex at all), as some of the questions are asked in a manner designed specifically to confuse a person as much as possible. If a subnetting question is especially confusing, don't waste a large amount of time on it.
If you know your stuff, you can miss some subnetting questions and still pass the exam. It would be better to miss some subnetting questions and answer all of the questions on the exam, then to run out of time on the exam because you spent too much time on a particularly confusing subnetting question. Remember, if you pass the exam by one point, you are just as Cisco certified as the math wizard who passed the exam with a score of 978.
If you aren't sharp at math, your goal should be to know your stuff well enough that you can miss some of the trickier subnetting questions on the exam and still pass comfortably. It can't be said enough times, you have to know your stuff.