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See the latest about a temporary release delay on new titles. Learn more. Subnetting is an absolutely critical component for CCNA success. Should you take the composite CCNA exam option, you need to be even better and faster at subnetting in order to achieve success. Then we will look at some shortcuts that can be used in the exam to ensure that we can solve the many subnetting challenges quickly and accurately. It is unacceptable, in my opinion, to master subnetting if you only know the shortcuts.

We need to fully understand why we are doing something in the network, and how it truly works before we can start seeking exam shortcuts. This article assumes you have a basic working knowledge of IP addressing and subnet masks. Also, this article assumes you can manually convert a binary octet to decimal, and vice versa. Why do we even need this concept of subnetting?

Well, we need to break up our networks into smaller networks all of the time. Breaking the network up allows the network to be more efficient and more secure and more easily managed. When you look at a Class B private address like You take the number of bits used for host addressing 16 , and you raise 2 to this power and then subtract 2. We need a calculator for this one because the number is so big!

It turns out that 65, can live in this network. So if we want to use the When we subnetwork, we play a balancing act. The formula for how many networks we can create is very similar. Let us study an example. If we have How many hosts can each of these subnetworks accommodate? Well, there are now 8 bits left for host addressing.

Let us stick with the example where we begin with the private IP address space of a Class B address of We examine our networking needs and realize that we need to accommodate different subnetworks in our organization.

This is perfect; we have the number of subnetworks that we need, plus a few extra that we can call upon when the network inevitably grows. What will the subnet mask be in this scenario? This mask will be the one that is used by all of the hosts in all of the different subnetworks. It is critical that we calculate this number correctly, of course.

Notice that our Class B address originally had 16 bits that made up the network ID portion. In this sample scenario we are going to borrow 7 bits. We can write out the 23 bits of the subnet mask now:. So what would the first subnetwork network ID be? Well, we know it will start To answer this in the longhand method, we write out the mask and the address from that octet in binary and do some analysis. Notice the first subnetwork will be We can use all zeros in the first 7 bit positions of the third octet, and we have a zero in the last bit position which is used to identify hosts.

What would the first host address be in this network? What would the broadcast address be for that network? To get this you fill all the host bits with a How about the last usable host address on this subnetwork? We will turn all the host bits to 1, except for the last one. What is the next subnetwork in this scheme? Well, let us turn one of those subnetwork bits on.

We will start with the least significant rightmost :. It is wonderful to see how all of this works longhand, but in the lab exam environment, we are VERY pressed for time.

We need powerful shortcuts. Here we will walk through my preferred shortcuts against the backdrop of sample exam questions. There are many different shortcut approaches that are all valid. This is the one that I prefer, but you might be taught another by some other Cisco trainer. Go with whatever works for you in the exam! What is the last usable address in the subnet of a host with the address Step 1 Upon arriving at my first subnetting question in the exam environment, I build a Powers of Two reference chart on the scratch paper Cisco provides.

Step 2 How many bits of subnetting are used in the fourth octet here? My Powers of Two chart tells me. This tells us the value that the subnets increment on. In our example it is Step 4 This host, with the address of The broadcast address for this subnet is one less than the next subnet of , so that is The last usable address is Your IT Junior Administrator has provided you with the address and mask of You Junior Admin has asked you to tell him how many hosts can be created on your subnet?

Therefore, I can see that there are 3 bits used for subnetting in that octet. This leaves 5 bits for host addressing. Initially, the subnetting related questions strike fear in the hearts of CCNA candidates. Sure enough, with study and practice, and the many shortcut methods that exist, these questions become the favorites in the Certification Exam environment. They can be solved easily and quickly, and candidates know they solved them correctly.

Page 1 of 1. Once this is done, we are going to look at some shortcuts that can be used in the exam to ensure that we can solve the many subnetting challenges quickly and accurately. NOTE There are many different shortcut approaches that are all valid. Related Resources Store Articles There are currently no related titles. Please check back later. Join Sign In. All rights reserved.


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