Whether you are prepping for a Cisco or College exam, chances are you will not be able to use a subnet calculator for your exam. Even if you can use a calculator, doing it by hand is faster once you know how! This post will show you how to subnet by hand, without memorizing a single chart.
Before we start, forget whatever you exam book, professor, or any other tutorial has tried to teach you. Subnetting does not need to be a four chapter process, or take two weeks of classes to teach. A few years ago when I was still in college I took an Internetworking course that spent an entire month on subnetting. Sure part of this was theory, but the majority was how to subnet! Guess what? By the end of that month I still had no idea how to properly subnet and had to use a calculator. With this method I was able to pick up subnetting in one hour and master it within a day or two.
Subnetting is easy, but we should define a few key points before we start working with subnets.
An IP Address consists of 32 bits, some for the network and some for the host. These 32 bits are split into 4 octets (separated by periods) to create a complete IP Address. There are three classes of IP Addresses.
Class A: 8 bits (the first octet) are reserved for the network, all others are for hosts.
Class B: 16 bits (the first two octets) are reserved for the network, all other are for hosts.
Class C: 24 bits (the first three octets) are reserved for the network, all other bits are for host usage.
We can visualize this a bit better by using a diagram:
NNNNNNNN. HHHHHHHH. HHHHHHHH. HHHHHHHH Class A
NNNNNNNN. NNNNNNNN. HHHHHHHH. HHHHHHHH Class B
NNNNNNNN. NNNNNNNN. NNNNNNNN. HHHHHHHH Class C
N = Network
H = Host
As stated earlier there are 32 bits in an IP address, separated into 4 octets. A period creates a boundary between each octet. If we add up each octet we come up with a few key numbers –
- 16. 24. 32
These numbers are important to remember as they will help us cut the amount of steps needed to find a subnet in half.
Typically if we are going to subnet or figure out which subnet a host belongs to we will be given an IP in CIDR or dotted decimal.
192.168.13.42/29 – CIDR Notation
192.168.13.42 255.255.255.248 – dotted decimal
Both of these are easily subnetted, but we’ll start with dotted decimal first as I find it slightly easier. Without further ado, let’s put this method to the test!
What subnet does 192.168.13.42 255.255.255.248 belong to?
Step 1. Find the first octect that does not have 255 in it (the last one in this case)
Step 2: Take that number (248) and subtract it from 256
256 – 248 = 8
8 is your block size. The block size is what separates the subnets (note: This number minus 2 will tell you how many addresses are available in the subnet).
The fourth octect did not have 255 in it, therefore we’ll be working with the fourth octect of the IP address.
Starting at 0 begin counting up by your block size (8). Do this in the identified octect until you pass the number in the question (42).
Step 5: Now that you have gone one network past the IP address in question go back one and you’ll know the network that the host belongs to. In this case the host belongs to network – 192.168.13.40
Pretty easy, right? Converting from CIDR Notation is nearly as simple!
What subnet does 192.168.13.42/29 belong to?
Step 1. Remember above, where I explained that if you add up each octet you’ll get 8, 16, 24, 32? This is where we’ll use that. The /29 indicates 29 bits are used for the network. Since 29 > 8,16,24 we’ll know that the network ends in the 4th octect. Take 32 – 29.
Step 2. 32 – 29 = 3. Next take 2^3 to get our block size. 2^3 = 8
Step 3. Just like step 3 for dotted decimal form we’ll begin counting up by 8 until we pass the address given to us in the question.
Step 4: Now that we’ve identified the next network passed ours, back up one to find out which subnet 192.168.13.42/29 belongs to – 192.168.13.40
There you have it. In just 4 steps we’ve figured out the subnet and you did not need to memorize a single table!
For subnetting practice check out - http://www.subnettingquestions.com/
I learned how to subnet via – http://techexams.net. The concepts in this post were outlined by LordFlasheart in 2008.