# Reverse Engineering an IPv4 Host Address

Want even more practice when it comes to IP version 4 subnetting? Enjoy this awesome Nugget from Keith Barker, Senior Technical Instructor with www.cbtnuggets.com. # Subnetting – What Mask to Use #### Overview:

Remember, it is totally worth repeating in this series of posts on subnetting – while practicing just how fast you can ace subnetting questions in the certification environment is always worthwhile and fun, you do want to make sure you understand exactly why you are going through the exercise to being with. You also want to be sure you understand how subnetting fundamentally works. This is often referred to as subnetting “longhand”. This is why a course like the CBT Nuggets IPv4 Subnetting – The Ultimate Guide is so critical.

#### ICND1 Sample Subnetting Question:

Your co-worker has decided upon use of the 172.16.0.0 address space for a section of your network. This section requires 14 subnets. What subnet mask will you recommend?

Step 1: I reference the Powers of Two chart I created on my scratch paper when I encountered the first question. The formula for the number of subnets you can create based on subnet bits is 2 ^ S where S is the number of subnet bits borrowed. From the chart I see if we “borrow” 4 bits we can create 16 subnets. This is two more than we need in this scenario, but that is great as it will allow for future expansion with ease.

2^7=128  |  2^6=64  |  2^5=32  |  2^4=16  |  2^3=8  |  2^2-=4 | 2  ^1=2  |  2^0=1

Step 2: Borrowing 4 bits beyond the Class B boundary results in 255.255.128+64+32+16 = 240. Our mask is 255.255.240.0.

# Subnetting – Hosts Per Subnet

#### The Joys of Subnetting Knowledge:

Remember, while practicing just how fast you can ace subnetting questions in the certification environment is always worthwhile and fun, you do want to make sure you understand exactly why you are going through the exercise to begin with. You also want to be sure you understand how subnetting fundamentally works. This is often referred to as subnetting “longhand”. This is why a course like the CBT Nuggets IPv4 Subnetting – The Ultimate Guide is so critical.

#### ICND1 Sample Question:

You have run the ipconfig command and discovered your IP address and mask are 192.168.20.102 and 255.255.255.224. How many hosts are permitted on your subnet?

Step 1: I reference the Powers of Two chart I created on my scratch paper when I encountered the first question. Adding 128 + 64 + 32 = 224. There are 3 bits used for subnetting and that leaves 5 bits for hosts.

2^7=128  |  2^6=64  |  2^5=32  |  2^4=16  |  2^3=8  |  2^2-=4 | 2  ^1=2  |  2^0=1

Step 2: The equation for the number of hosts per subnet is 2^h – 2 where h is the number of host bits. From the chart I see that 2^5  = 32. 32-2 = 30 hosts per subnet! Too easy!

As always, let us know in the comments if you have a quicker approach.