I wanted to make a subnetting tutorial that showcases my absolute favorite shortcut for finding the network address given an IP and mask.
This video shows in my opinion the fastest way to find the network address where an IP “lives”. This comes in handy when troubleshooting on the CCNA exam. Check the Video out below:
How to Subnet: Finding the Network Address
- Find your ‘interesting’ octet.
The interesting octet is the octet where the subnet bits end and the host bits begin.
- List out the bit values [128, 64, 32, 16, 8, 4, 2, 1] and draw a line to the right of the last subnet bit. Ex: a /10 would draw the line between 64 and 32, because you need to borrow 2 bits from the second octet to make a /10 subnet (10 subnet bits total).
- Look at the value of the ‘interesting octet’. Let’s say we are looking at 10.128.45.3 /10. Our interesting octet is the second octet, and the value there is 128.
- If you can subtract the value of the subnet bit from the value in your ‘interesting octet’, do so. Do this until you either reach 0, cannot subtract without going negative, or you reach your “dividing bar” where the subnet bits stop and the host bits begin.
For every subnet bit that you can subtract, ‘turn that bit on’. You can write a 1 below the value to keep track.
- When you complete that process, add the value of every bit that you ‘turned on’. This is the value of the network address that the IP you were studying live in.
Look at the picture below for our 10.128.45.3 /10 example
If these instructions don’t make sense, check out the video above!
Disclaimer: Know the Binary
You don’t have to be a binary master, or even use it ever again once you understand the basics. Click here to check out a great introduction to ‘bit values’ and IP in binary.
Links to OSPF over GRE Tunnel Lab, Subnet Game, and More
Check out my Python3 CCNA Subnetting Game
Do you enjoy my content? I enjoy making it 🙂 Here is a link to my favorite CCNA Book. This is an amazon affiliate link, so if you purchase through the link it will help me keep the site hosted.
This CCNA study guide is for the old version of the test. HOWEVER, the information in it is invaluable. The chapter on subnetting is brilliant, and it was the only thing that broke it down in a way I understood. The information in this book is still valid. It will not prepare you completely for the new model exam, but if you want a good book for the shelf this is discounted now that the new exam is out.
Many people complain that the Odom book is dry, but it really goes in depth and I owe the strength of my foundation in networking, and quite frankly, my career to this book. I carry it with me every where I go out of principle. It is not perfect, but it’s d*mn good.
Until next time, thanks for reading guys 🙂