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IP Subnetting is a process of breaking a large network IP range to the small & reliable subnets. Subnets means the subnetwork of a large network. Subnet mask contains network & host portion. We acquire host portion to the network & perform subnetting. We know that one large network is not good thing so, we reduce the network congestion in our organization using subnetting.

Lets see an example of a large network :- 192.168.10.1/24 of class c default network, so the subnet mask is 255.255.255.0, now convert this decimal value into binary.

  255          .  255          .  255         . 0

11111111 . 11111111 . 11111111 . 00000000

            Network Bit                           Host Bit

All ones are on bits & represent network bit & all zeros are off bits represents as host bit. every octate have a maximum number is 255 in decimal, so, as per the above example there are 255 hosts available in the network range. suppose if we required only 25 hosts then our (255-30 = 225) IP will be waste. 

We need subnetting to Organized our network, Reduce congestion & Prevent unused IP addresses.

Yes we will do subnetting but before that lets understand one more terms is CIDR. 

Classless Inter-Domain Routing (CIDR) :-

CIDR is a slash (/) notation. When you receive any block of IP like 192.168.10.1/24 it is telling your subnet mask, slash notation means the on bits or the bits turns into 1's. In the block of 192.168.10.1/24, 24 bits are turned on. 

There are 4 octate in ip address, 8 bits in each so total number of  bits would be 8*4 = 32 . When we sunet any block there are 2 ip addresses are reserved first one for the Network address and Last one for Broadcast address, in the view of above till the /30 we have 4 ip addresses (2 are Network & Broadcast, other 2 are only valid host ip) so, practically to build a network we need 2 or more hosts & IPs. So, practically we can use only till /30 CIDR value.

Find the below subnet mask value for each slash notation CIDR value.

You can find out subnet mask for any slash (/) notation... as for example for /27 we have to find subnet mask so, we have already read IP class in last post What is IP address 

 if not please refer. So, /27 means there are 27 bits are on out of 32 like this:-

11111111.11111111.11111111. 1110 0 0 0 0

here we can see the power of 2 means 2^x . in last octate there are 5 bits are off means 0...... we will subtract the value from the 256.

11111111.11111111.11111111. 111 0    0    0    0    0

                                                       🠛    🠛    🠛    🠛    🠛

                                                 2⁵ = 32     2⁴     2³         2²   2¹= 2           

We will count the power of two from right to left so the last off  bit  power of 2 is 32 so, minus this value from the 256. It will be 256-32= 224 so the subnet mask will be 255.255.255.240.

             

Now let's do subnetting in very easy steps.....!!! Think about these five points before subnet any IP block.

1. How many subnets does the chosen subnet mask produce?
2. How many valid hosts per subnet are available?
3. What are the valid subnets?
4. What’s the broadcast address of each subnet?
5. What are the valid hosts in each subnet?

Now let's  subnet a block e.g 192.168.10.1/28, here /28 denote the total number of on bit out of 32 bits means total 1's. 

We are going to acquire host bit to the network bit since, it is a class C ip address so by default 24 bit allowed for network portion & we are going to acquire 4 more bits from the host portion, you can see in the highlighted text we have to check values from the portion from where we are acquiring bits.So................

    1. How many subnets does the chosen subnet mask produce?

We will use formula 2^x here x is the on bits or the masked bit. So, in our example check the highlighted portion there are 4 masked bits or ones so it is simple to find the number of subnets is... 

2⁴ = 2*2*2*2 =16 Subnets.

    2. How many valid hosts per subnet are available?

We will use formula 2^y - 2 here y is off bit or all zeros.So, in our example check the highlighted portion there are 4 off bits or zeros so it is simple to find the number of  hosts per subnets is... 

2⁴ – 2 = 14 hosts per subnets

    3. What are the valid subnets?

We can determined valid subnet by using formula 256-subnet mask so, it is 256- 240  (Subnet mask of /28 is 255.255.255.240) = 16 so, block starts with 0 then add 16 until 240.so it will be 0,16,32,48,64,80,96,112,128,144,160,176,192,208,224,240. 

    

    4. What’s the broadcast address of each subnet?

The broadcast address will be the number just before the next subnet number or the last number of current subnet block. lets check in our example we have already find the subnets in point number 3 so ,check the broadcast address of subnet 16 is 31, because the next subnet is 32..... broadcast address of 32 is 47.

    5. What are the valid hosts in each subnet?

Valid hosts are always a group of numbers between the subnet & broadcast address, we have already find the both in point number 3 & 4 so lets find the valid hosts for first block 0 to 15 the valid hosts will be 1-14 means there are valid host range is 14, as it is for the next block 16 to 31 valid hosts will be 17-30............. yes its easy!!!!!

This is how we can subnet easily & find all the required details. For this class c subnet find all the details in below mentioned table:-