The art of Subnetting
Subnet Introduction
To begin, lets talk about subnets. What is the purpose of a subnet?
subnets are for segmentation and division of an IP scope. For example you have 40 PCs, and 20 Printers. You can divide your IP address scope in a way that facilitates your 40 PCs on one subnet, and 20 Printers on the other subnet. This creates segmentation of the network and is great for network security as well (we will get to this later on).
Example I
You have the IP scope of: 192.168.1.0/24 (click here if you are having trouble with understanding /24, /25, /26, etc.).
You Have:
- 50 PCs
- 40 Printers
- 10 APs
How would you solve this?
When creating subnets, best practice is to always start with the one with largest amount of devices. In this example it would be the 50 PCs. (Check out How to Subnet here for deeper instructions)
Then you will follow up with the second network with largest amount of devices. In this case the 40 Printers.
Lastly you will end it with the 10 APs, or the smallest network.
So if you have 192.168.1.0/24, you have exacly 254 usable IP addresses. Now if we want to only use 50 addresses for our PCs, we will need to divide this /24 to something more useful.
A /26 would be better for this, as you would have 12 addresses left over. If you would have used /24, you would have 204 addresses left over (wasting ip address space).
for the 40 Printers we would use a /26 as well. and for our 10 APs we would use the /28.
Cheat Sheet:
- /24 = 254 usable IPs
- /25 = 126 usable IPs
- /26 = 62 usable IPS
- /27 = 30 usable IPs
- /28 = 14 usable IPS
Something to keep in mind
from the above example, just know that you also have to keep in mind of how much IP space you are using. Since we have used two /26s, and one /28. Even if we are not using all the IP space and each of those subnets, you must count the entire subnet.
Lets dive deeper.
If you have two /26s, then you are already using 128 addresses. Now add our /28 that uses 16 addresses. As of this moment we are using a total of 144 addresses.
With that information we know that we now only have 256 – 144 addresses. Which means we have 112 addresses left over.
Cheat Sheet #2
- /24 = 256 addresses 254 usable IPs
- /25 = 128 addresses 126 usable IPs
- /26 = 64 addresses 62 usable IPS
- /27 = 32 addresses 30 usable IPs
- /28 = 16 addresses 14 usable IPS
Example II
You have IP Scope: 10.0.0.0/25
You need to divide this address for the following devices:
- 30 PCs
- 24 Mobile Phones
- 12 Printers
- 9 APs
Again we start with the largest network first, this will be the 30 PCs. Second we go for second largest, 24 Mobile Phones. Then we go for the 12 Printers, and finally we go for the 9 APs.
- The 30 PCs become a /27
- The 24 Mobile Phones become a /27
- The 12 Printers become a /28
- and the 9 APs become a /28
Now that we are done with that, we need to check that we did not go over our IP space. So lets check:
- PCs /27 = 32 addresses
- Mobile Phones /27 = 32 addresses
- 12 Printer /28 = 16 addresses
- 9 APs /28 = 16 addresses
This would mean we are using a grand total of: 96 Addresses. Since we are using 10.0.0.0/25, this means we are perfectly in our IP space. (/25 = 128 addresses).
Final Challenge
Now that you seem to be getting the hang of subnetting, there is another step. We now need to define each Subnet Network. What do I mean by that?
Here is an example:
PCs are a /27, and our address starts at 10.0.0.0/25. We have 128 total addresses, so our first address starts at 10.0.0.0. Since this is a /27 we have 32 usable IP address, here is where it starts making sense. /27 has 32 usable IP address, but in reality /27 has 34 addresses in total. The first address 10.0.0.0 is our network address, and our last address 10.0.0.33 is our broadcast address.
With all this math out of the way, now we can Identify our first subnet network which is:
PCs = 10.0.0.0-33 /27
Now lets try the next one. Mobile phones /27, first address if 10.0.0.34, and last address is 10.0.0.63. Are you starting to see the pattern?
Mobile Phone = 10.0.0.34-63 /27
I will let you finish solving for the last two subnet networks (12 Printer, and 9 APs). I will post the answers on my next subnet post which will be called Subnet Tutorial II. Until then continue practicing subnetting. This is a concept that once you understand, will help you alot in networking.
Study Tip
The only way to get good at subnetting is to actually attempt it. When I was learning, a good tool that helped was using an AI Chat.
To do this you must configure your question like this. “Can you ask me to find the subnet of an ip? Tell me if a get the answer correct, and if I get the answer wrong, explain to me why I am wrong.“
A question like this will give you INFINITE question possibilites. This is also a good way to prepare for exams.
Conclusion
This is still the basics, subnetting gets a little deeper than this. I will continue covering this topic in the future. Some of the things I will talk about are like finding the broadcast address, and the network address. Also I will cover subnet wildcards, which are used a lot in ACLs (Access Control List).
I have also started another post that focuses in CCNP SCOR 350-701 study material. Though if you are barely starting out, learning the basics will help you later.
Until then continue studying!