How To Subnet The Easy Way
For the past three months I’ve been studying for my CompTIA Network+ and Cisco Certified Network Associate (CCNA) certification exams. Recently I have taken and passed my Network+ exam but I’m still preparing for the CCNA as this exam is a lot more comprehensive, especially in subnetting. I took the N10-007 and scored a 747 on my first attempt. Subnetting is one of the most important topics covered in Network+ and Cisco exams so it’s paramount that you understand how it works. During my studies I came across a few resources that helped me master the concept of subnetting and solve any subnetting problem within one minute. Although it can take some time and be tough to grasp, you will be able to master subnetting if you follow these steps and practice.
- SUBNETTING IS LIKE GOING TO THE GYM, YOU JUST HAVE TO DO IT TO GET BETTER!
Before you read this you should already have a basic understanding of IPv4 and binary math. If you don’t know binary you can watch my Learn IPv4 and Binary Math video, which will bring you up to speed. Alright, in order to make subnetting simple you need to memorize two basic charts and simple math, which I have posted below. You should be able to create this chart from memory after some time.
Chart #1 displays your CIDR notations, their corresponding subnet masks, and the number of hosts and networks that it will allow. This second chart is just your 8 bits written out, writing this out will help you quickly find your network ID. I only wrote down /1 through /8 to save time but you can do all /32 if you need to. You would write four columns with 8 rows each. So the next column would be /9 through /15 then the third would be /16 through /24.
You will also need to know the powers of 2. This will help when we get into VLSM.
2 4 8 16 32 64 128 512 1024 2048 4096 8192
Finding The Magic Number
Let’s take a look at this sample subnetting question
What are the network address, broadcast address, and the subnet mask for a host with the IP Address below?
IP Address: 18.104.22.168 / 21
First we need to ask ourselves, what class is this IP address in? Since it falls between the 192-224 range we know that it’s a class C IP address. Class C IP addresses are a /24 by default therefore we know that it has already been subnetted. We could do this the long way and do a bitwise anding but using your chart will save you time when taking the exam. Now we need to refer back to our chart. This will give us our subnet mask and magic number. In this case our subnet mask is 255.255.248.0 and our magic number is 8 so we know that the networks are going up by 8.
Now we list out our multiples of 8
8, 16, 24, 32, 40, 48, 56. Since 49 falls in between 48 and 56 we know that our network address is 22.214.171.124
To find our broadcast address we just subtract 1 from the next network so the broadcast address is 126.96.36.199 (Because 188.8.131.52 is the next network).
Our subnet mask is 255.255.248.0 /21 (/21 indicates 21 bits turned on)
We will examine an example problem for a Class B.
Enter the maximum number of valid subnets and usable hosts per subnet that you can get from the network 172.19.0.0 255.255.255.240.