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How to Subnet for IPv4 Class C Addresses.

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In this article I will be showing you how to subnet the easiest way possible for IPv4 (Internet Protocol version 4). This article does not cover IPv6.

Keep in mind that subnetting requires lots of practice and time.

How to subnet in a detailed way without any type of introduction that makes you feel bored.



I know I have promised you not to read too much, but let's be familiar with some concept before move on.


What is a subnet?
Subnetting is basically just a way of splitting a TCP/IP network into smaller, more manageable pieces. 


What its purpose?
The main purpose of subnetting is to help relieve network congestion such as traffic.


What's the benefit?
Prevents Unnecessary Broadcasts, Increases Security Options, Simplifies Administration and last but not least Controls Growth.


What is binary?
Binary describes a numbering scheme in which there are only two possible values for each digit: 0 and 1
In computer electrical terms 1 means ON 0 mean OFF.


IPv4 Address Classes?


Class A addresses are used for very large networks and always start with a leftmost bit being a zero.

A class A network can hold as many as 16,777,214 hosts.

Class Private IP Address Range Subnet Mask
A 10.0.0.0 to 10.255.255.255 255.0.0.0


Class B These addresses are used for medium sized networks and always start with the leftmost 10 bits. lass B network can hold as many as 65,534 hosts.

Class Private IP Address Range Subnet Mask
B 172.16.0.0 to 172.31.255.255 255.240.0.0


Class C These addresses are used for smaller networks, the one you have at home and always start with the leftmost bits 110. Each class C network can only hold up to 254 hosts.

Class Private IP Address Range Subnet Mask
C 192.168.0.0 to 192.168.255.255 255.255.0.0


What's VLMS?
Variable Length Subnet Masking "VLSM" is a technique that allows network administrators to divide an IP address space into subnets of different sizes, unlike simple same-size Subnetting. In a way, means subnetting a subnet


**End of the introduction**



Now, looking at the diagram, we have three LANs connected to each other with two WAN links. The first thing to look out for, is the number of subnets and number of hosts. In this case, an ISP allocated 192.168.1.0/24. Class C


R1 = 50 host
R2 = 20 host

R3 = 90 host

2 WAN links.


We will try and subnet 192.168.1.0 /24 to sooth this network which allows a total number of 254 hosts I recommend you get familiar with this table below. Or at least learn the binary values.


NOTE: As a first rule we need to start using the network that require the more hosts until we reach the lowest.


Let's begin with R3 with 90 hosts.


We are borrowing 1 bit to the value 128 on the table above, why? Remember, we need 90 hosts therefore 128 fits in the range. Now, our subnet is 192.168.1.0 /24 but after we've found an extra Bit we can do the follows:


1- Convert to binary our old subnet mask adding the new borrowed bit.
     (Bits) 11111111.11111111.11111111.1000000 = 255.255.255.128


Our new subnet is 192.168.1.0 /25, why 25? If you count the Bits you will notice that you have a total of 25.
So here is the outcome of what we did.


R3 subnet: 192.168.1.0 /25

Network: 192.168.1.0 

First IP: 192.168.1.1

Last  IP: 192.168.1.126

Broadcast: 192.168.1.127 


Easy right? now keep in mind that 128 it's our next network for the following subnet below.

R3 has been completed.



Let's work with R1 50 hosts:


As we were working on R3 we know for fact that our next network for R1 is 128 and therefore we have our network ID and first IP.

Network: 192.168.1.128
First IP: 192.168.1.129


Now, we need to find where this network ends. R1 require 50 hosts. If we look at the table above we know that 64 will give us the right range for the mentioned hosts, therefore we need to borrow 2 bits. so let's add more bits to our old subnet mask.


1- Convert to binary our old subnet mask adding the new borrowed bit.
     (Bits) 11111111.11111111.11111111.1100000 = 255.255.255.192


Our new subnet is 192.168.1.128 /26, why 26? If you count the Bits you will notice that you have a total of 26.
So what's next? we need to find our incremental.
"Always remember that our incremental is the last Bit you have taken borrowed". In this case 64.


So let's do the math 128+64=192


R1 subnet: 192.168.1.128 /26

Network: 192.168.1.128

First IP: 192.168.1.129

Last  IP: 192.168.1.190

Broadcast: 192.168.1.191


Getting in there right? now, keep in mind that 192 it's our next network for the following subnet below.

R1 has been completed.




Let's work with R2 20 hosts:



Again, as we were working on R1 we know for fact that our next network for R2 is 192 and therefore we have our network ID and first IP.

Network: 192.168.1.192
First IP: 192.168.1.193


Now, we need to find where this network ends. R2 require 20 hosts. If we look at the table above we know that 32 will give us the right range for the mentioned hosts, therefore we need to borrow 3 bits. So let's add more bits to our old subnet mask.


1- Convert to binary our old subnet mask adding the new borrowed bit.
     (Bits) 11111111.11111111.11111111.1110000 = 255.255.255.224


Our new subnet is 192.168.1.192 /27, why 27? If you count the Bits you will notice that you have a total of 27.
So what's next? we need to find our incremental.
"Always remember that our incremental is the last Bit you have taken borrowed". In this case 32.


So let's do the math 192+32=224


R1 subnet: 192.168.1.192 /27

Network: 192.168.1.192

First IP: 192.168.1.193

Last  IP: 192.168.1.1.222

Broadcast: 192.168.1.223


Even easier right? now keep in mind that 224 it's our next network for the following subnet below.

R2 has been completed.





Let's move onto WAN 1 (2 hosts) 



Same methodology, my old subnet is 192.168.1.0 /24, we need 2 host for each wan link right?. So our magic number will be 4 and we need to borrow 6 bits.


1- Convert to binary our old subnet mask adding the new borrowed bit.
     (Bits) 11111111.11111111.11111111.11111100 = 255.255.255.252


Our new subnet is 192.168.1.224 /30, why 30? If you count the Bits you will notice that you have a total of 30.
So what's next? we need to find our incremental.

"Always remember that our incremental is the last Bit you have taken borrowed". In this case 4.


So let's do the math 224+4=228


WAN link subnet: 192.168.1.224 /30
Network: 192.168.1.224

First IP: 192.168.1.1225

Last  IP: 192.168.1.1.226

Broadcast: 192.168.1.227





Let's move onto WAN 2  (2 hosts) 



Same methodology, my old subnet is 192.168.1.0 /24, we need 2 host for each wan link right?. So our magic number will be 4 and we need to borrow 6 bits.


1- Convert to binary our old subnet mask adding the new borrowed bit.
     (Bits) 11111111.11111111.11111111.11111100 = 255.255.255.252


Our new subnet is 192.168.1.228 /30, why 30? If you count the Bits you will notice that you have a total of 30.
So what's next? we need to find our incremental.

"Always remember that our incremental is the last Bit you have taken borrowed". In this case 4.


So let's do the math 228+4= 232


WAN link subnet: 192.168.1.228 /30
Network: 192.168.1.228

First IP: 192.168.1.1229

Last  IP: 192.168.1.1.230

Broadcast: 192.168.1.231




Phew! That was all. Now we know what subnet need each router we can start using it's respective IP addresses.

R3: 192.168.1.0 /25

R1: 192.168.1.128 /26

R2: 192.168.1.192 /27

WAN1: 192.168.1.224 /30

WAN2: 192.168.1.228 /30


I hope this is been informative for all who wants to know how to subnet.
Later in the future I will be adding more post with class A and B, stay tuned


If you want to practice more I 100% recommend this website https://www.subnetting.net/Tutorial.aspx


This website will help you to practice subnet until you learn. Also they can teach you CCNA and CCNP for a minimum cost of 5 bucks a month what a deal right?


I learned a lot with this website and if you have any question they'd help you with your question to reach your goals.


Thank you for reading and if you enjoyed my article please endorse!

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Expert Comment

by:James Bunch
This article is amazing. I am studying for the Network+ N10-006 test right now and was able to use this information along to correct mistakes I was making previously in trying to determine Subnets and ranges of given IP address/CIDR on the practice tests. This makes it easier to keep the math simple in my head as well as to follow how to finish the resolution of the problem without reworking parts of the scenario. Great work and thank you for sharing it with us!
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