A Subnetting Cheat Sheet – Why You Need One

subnetting cheat sheetA cheat sheet is a great way to study for an exam and get all the important information in one sheet. In certification exams, students are not allowed to take notes into the exam room, but you can jot down information during the test to quickly look up information while you take the test. By doing this, the student can save a lot of time and stress during the exam when time is at a premium.

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If you already knows the material, why construct a cheat sheet? Well, knowing the information is only half the battle. The problem is there may be, for example, ten or fifteen questions on IP subnetting. That would mean recalculating for every question. However, if the student has invested time creating just a rough cheat sheet, then it is simply a case of looking up the information.

So how does the cheat sheet work?

Consider the example below:

The power of the cheat sheet is in the fact that all the relevant information is available as you answer questions, which allows for quick and efficient retrieval even under stressful circumstances as in an exam.

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Here are some subnetting questions where you can refer to your cheat sheet for answers:

Question 1: Find the maximum number of valid subnets and hosts per subnet that you can get from the network 192.168.232.0/24:

Question 2: Find the first valid host on the network 172.23.236.100 255.255.255.252:

Question 3: Which subnet mask would you use for the 172.29.0.0 network, so you can get 3210 subnets and 10 hosts per subnet?

Lets take a look at these questions starting with question one. The result expected is the number of subnets and valid hosts that are available with a /24 subnet mask. Now, one way is to calculate using the formula “subnets = 2^n” where n is the number of bits available. We can calculate n by subtracting 24 from the full mask 32, which is (32-24) = 8, 2^8 or 256. However, the question asks for valid subnets and hosts, so you must subtract the subnet id and the broadcast addresses. So, the end result is 256-2=254 hosts. So, there is one subnet (all bits are used for hosts) and 254 valid host addresses.

OR

You can look at the cheat sheet above, look at Network C and the /24 mask and read the results. The only information you need to know is whether to search for network class A, B or C.

On to question 2. Again, this can be calculated by determining the block size then counting up from zero until the correct subnet is found. For example, 256-252 = 4. The subnet is given as 172.23.236.100, so the first valid address is 172.23.236.101. The other option is to use the cheat sheet, find the subnet mask 255.255.255.252 for the class B address and determine immediately that there are only two valid hosts. Therefore, the first must be 1722.23.236.101

The third question is where the cheat sheet is really useful. Simply consult the table and look for a mask that meets the requirements. It is a class B address with a /28 mask.

ipcheatsheet1

This particular cheat sheet is very thorough, but it’s probably difficult to write down all of this information during test time. However, you can quickly recreate a similar table using just the salient information.

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