How to use your instant cheat-sheet – Part 1

A few weeks back I shared my ideas for an instant cheat-sheet with all of you.  I then told you how I was able to successfully recreate it and use it on the CCENT exam (See the post “Observations from taking the CCENT exam - Part II”).  I can’t stress this enough; this practice is all perfectly legal and is very effective at raising your score on the exam.  However, you need to know what the information on the cheat-sheet stands for, you need to know how to apply the information from your cheat-sheet on the exam, and you need to know how to recreate the cheat sheet in the short period of time that you have before the exam begins.
In today’s post I want to explain Table 1 on the cheat-sheet. I will explain the other tables and show you how to use the tables in future posts.

Table 1: Conversions and Jump Numbers

 

 

 

 

You should be well aware of the top row, the decimal value of each bit in an octet.  The bit on the right is bit 0 and is worth “1” which is derived from base2 numbering, 2⁰  or 2 raised to the zero (0) power. As you can see, the value of the bits double as you move left and the power of 2 increases (2¹, 2², 2³, etc).  That’s the beauty of base2. The values also represent the Jump Number, or number of addresses that make up each network derived from the original classful network (more on this later).

Now that we’ve established the meaning of the top row, we can now examine rows 2-4. Rows 2-4 represent the bits in octets 2-4 and the possible subnet masks in prefix (slash) notation. This is extremely easy to recreate since all you’re doing is counting, starting at the 9^th bit position. Note: you should never see a subnet mask that is less than 8, unless it’s a special address or illegal.

What does all of this mean? Recall that every IP address has two parts; the network portion and the host portion. A mask of /20 means that the first 20 bits (from left to right) are network bits, and the 12 bits that remain are host bits.  In most IP addressing questions we are interested in resolving an address to the network portion of the address.  All of the host bits then become zeros (0). For instance, the address 172.16.123.88 /16 becomes 172.16.0.0 when you apply the subnet mask. The /16 implies that there are 16 network bits (from left to right) and the remaining 16 bits are host bits and they are filtered (zeroed) out.

The bottom row represents the other type of mask that you will see (dotted-decimal).  The number is calculated based on the sum of the network bits (from left to right) in a given octet. A mask of /20 means the first 16 bits in octets 1 and 2 are all part of the network, along with the first four bits of octet 3.  If you add the decimal values of the first four bits of octet 3, the sum is 240 (128+64+32+16). Therefore the mask /20 converts to 255.255.240.0. Remember, the remaining bits, bits 21-32 are all host bits and are changed to zeros (0) when determining the network address.

In Part 2 we will see how we can apply this to a problem.

-Jim (revised 6/19/14)