What is Subnetting and how to do Subnetting ?

           Before going to Subnetting, If you haven't studied about IP Address, I recommend you to go through IP Address What is an IP Address.

What is Subnetting ?

• Subnetting is nothing but Dividing a Classful network in many Classless network.
• Subnetting is mainly based on Subnet Mask.

What is CIDR ?

• Classless Inter-Domain Routing
• In simple words, It represents the alternate way of representing the Subnet-mask.
• 255.255.255.0 - /24
• 255.255.255.0 can be represented as /24
• 24 represents the number of bits , Each Octet is 8 bit,  Each Full Octet is represented by a Value of 255 , so each 255 value is equal to 8, Hence 8+8+8+0 = 24
• 255.255.255.128 - /25
• 255.255.255.128 can be represented as /25
• 25 represents the number of bits , Each Octet is 8 bit,  Each Full Octet is represented by a Value of 255 , so each 255 value is equal to 8, 128 is fetched by the value of  27 , which is the first bit value to be added for Every Octet, so the value of one is added. Hence 8+8+8+1 = 25

When Subnet mask does not belongs to any classful Subnet as 255.0.0.0 , 255.255.0.0 or 255.255.255.0 , then the increment value will be like this..

How to calculate Subnetting ?

Example 1 : 

• Consider this IP Address : 192.168.1.25 / 28
• CIDR of /28 can be divided into 28 = 8+8+8+4 = 28
• From the above table 4 = 240 (128+64+32+16=240)
• Subnet Mask : 255.255.255.240
• 256 is the total number of Bits (From 0 - 255 )
• 256 – 240 = 16 -- > Each network has 16 IP Address ( or 24 =16 )
• 0 , 16 , 32 , 48 ,…..
• Our IP Address is 25 ,
• 25 falls between, 16 and 32.

The Subnetting Solution for the Network 192.168.1.25/28 is ...

• Our Network Address : 192.168.1.16 / 28
• First IP Address : 192.168.1.17
• Broadcast IP Address : 192.168.1.31
• Last IP address : 192.168.1.30

Example 2 :

• Consider this IP Address : 192.168.45.25 /21
• 21 = 8+8+5+0 = 21
• Subnet Mask : 255.255.248.0
• 256 – 248 = 8 -- >   211  = 2048 , Total IP Address used in this Network is 2048
• (28+3 = 211 )
• Where 3 comes from 3 octet which is 5 , (8–5=3 ) or (32-21=11)
• Where 32 is total number of bits for an IP address.
• 211  = 2048 , Total IP Address used in this Network is 2048
•  2048 , 4096, ….
• It will be difficult to calculate for such a high number, But it can be done easily by Octet level.
• 3rd Octet will be seperated by each 8 values. (3rd Octet will be as 0,8,16,24,32,40,48… ) 
• Our IP Address is 45.25 (Since 21 falls by 3rd octet )
• 45 falls between 40 and 48.

The Subnetting Solution for the Network 192.168.45.25 /21 is ...

• Our Network Address : 192.168.40.0 / 21
• First IP Address : 192.168.40.1
• Broadcast IP Address : 192.168.47.255 (Since Network Segment will be 192.168.48.0/21 )
• Last IP address : 192.168.47.254

Example : 3 

• Consider this IP Address : 172.16.95.10 /16
• 16 = 8+8+0+0 = 16
• Subnet Mask : 255.255.0.0
• Total usable IP’s – 216  (32-16 [8+8] = 16)
• Since the Subnet mask is 255.255.0.0 , It is not Subneted.
• Our IP Address is 95.10 (Last two Octet will be Entirely used for Host Segment)
• 95 falls between 0 and 255.

The Subnetting Solution for the Network 172.16.95.10 /16 is ...

• Our Network Address : 172.16.0.0 / 16
• First IP Address : 172.16.0.1
• Broadcast IP Address : 172.16.255.255
• Last IP address : 172.16.255.254

You can also Look into my Video Tutorial:

That,s All About Subnetting!!!

If you have any queries kindly give a comment