How to use your instant cheat-sheet – Part 2

In today’s installment we will look at a typical question you might find on a CCENT or CCNA exam, and we will use Table 1 from my previous post (See: How to use your instant cheat-sheet – Part 1) to help answer the question.
In the following figure we see a typical troubleshooting question that I like to refer to as a “what’s wrong with this picture” question.  In this scenario, Host C cannot communicate with hosts A and B. What is most likely the problem?

 

 

 

 

 

 

 

The first thing I would do is check to see if host C is properly configured and able to communicate with its default gateway.  If host C can’t talk to its default gateway, I can’t talk to hosts on another network.  Think of the default gateway like the only door in a room.  If you can’t open the door, you’re not getting out of the room.

Let’s use Table 1 below to see if Host C and the default gateway are on the same network.
 

 

 

 

 

 

Host C has a mask of /27.  Looking at our table we see that /27 lines up with the Jump Number 32.  That means that the fourth octet of Host C’s network address is an increment of 32, i.e. 0, 32, 64, 96, etc.. . Therefore, Host C resides on the 172.16.31.64 network, which has a valid host range of 172.16.31.65 through 172.16.31.94 (if you’re not sure how I arrived at that, ask). Note: 172.16.31.95 is the broadcast address and 172.16.31.96 is the network address of the next network. 

Now let’s look at the default gateway address, 172.16.31.93 /28.  Looking at our table we see that /28 (see below)lines up with the Jump Number 16.  That means that the fourth octet of the gateway network address is an increment of 16, i.e. 0, 16, 32, 48, 64, 80, 96 etc.. . Therefore, the default gateway resides on the 172.16.31.80 network, which has a valid host range of 172.16.31.81 through 172.16.31.94.

 

 

 

 

 

The key here is that Host C and the default gateway are on different networks; therefore they are unable to communicate with one another.  Any time Host C wants to communicate with a host on another network it needs the cooperation of the default gateway (think of the gateway as a type of doorman).  Of course realizing that the masks were different from the start would have been enough to answer this question; however my intent was to show you how to use the table.

By the way, they will generally show one mask in prefix notation and the other in dotted-decimal notation as depicted below.

 

In this case we could use the table to convert prefix notation to dotted-decimal notation, or vice versa, and then compare the two. For example:

/27 maps to a dotted-decimal mask of 255.255.255.224, and /28 maps to a dotted-decimal mask of 255.255.255.240.

In Part 3, I will introduce the other two tables that should be included on your cheat-sheet and I will work through another problem.

-Jim (revised 6/21/14)