## Subnetting in Networking-

Before you go through this article, make sure that you have gone through the previous article on Subnetting.

We have discussed-

• Subnetting is a process of dividing a single network into multiple sub networks.
• The number of sub networks created depends upon the requirements. Subnet mask is a 32 bit number which is a sequence of 1’s followed by a sequence of 0’s where-

• 1’s represent the global network ID part and the subnet ID part.
• 0’s represent the host ID part.

## How to Calculate Subnet Mask?

• By setting all the bits reserved for network ID part and subnet ID part to 1.
• By setting all the bits reserved for host ID part to 0.

Now, let us discuss some examples on how to calculate subnet mask for any given network-

## Example-01:

Consider we have a network having IP Address 200.1.2.0. Clearly, this IP Address belongs to class C.

In class C-

• 24 bits are reserved for the Network ID part.
• 8 bits are reserved for the Host ID part.

• By setting the first 24 bits to 1.
• By setting the remaining 8 bits to 0.

= 11111111.11111111.11111111.00000000

= 255.255.255.0

## Example-02:

Consider a single network having IP Address 200.1.2.0 is divided into 4 subnets as shown- Now, let us calculate the mask subnet for each subnet.

For each subnet-

• 24 bits identify the global network.
• 2 bits identify the subnet.
• 6 bits identify the host.

For each subnet, subnet mask is obtained-

• By setting the first 26 bits to 1.
• By setting the remaining 6 bits to 0.

= 11111111.11111111.11111111.11000000

= 255.255.255.192

### NOTE

In fixed length subnetting,

All the subnets have same subnet mask since the size of each subnet is same.

## Example-03:

Consider a single network having IP Address 200.1.2.0 is divided into 3 subnets as shown- Now, let us calculate the subnet mask for each subnet.

### For Subnet A-

For subnet A-

• 24 bits identify the global network.
• 1 bit identify the subnet.
• 7 bits identify the host.

For subnet A, subnet mask is obtained-

• By setting the first 25 bits to 1.
• By setting the remaining 7 bits to 0.

= 11111111.11111111.11111111.10000000

= 255.255.255.128

### For Subnet B And Subnet C-

For subnet B and subnet C-

• 24 bits identify the global network.
• 2 bits identify the subnet.
• 6 bits identify the host.

For subnet B and subnet C, subnet mask is obtained-

• By setting the first 26 bits to 1.
• By setting the remaining 6 bits to 0.

= 11111111.11111111.11111111.11000000

= 255.255.255.192

### NOTE

In variable length subnetting,

All the subnets do not have same subnet mask since the size of each subnet is not same.

• Subnet mask is used to determine to which subnet the given IP Address belongs to.
• To know more, Read here.

## Note-01:

• Default subnet mask for Class A = 255.0.0.0
• Default subnet mask for Class B = 255.255.0.0
• Default subnet mask for Class C = 255.255.255.0

## Note-02:

• Network size is the total number of hosts present in it.
• Networks of same size always have the same subnet mask.
• Networks of different size always have the different subnet mask.

## Note-03:

• For a network having larger size, its subnet mask will be smaller (number of 1’s will be less).
• For a network having smaller size, its subnet mask will be larger (number of 1’s will be more).

## Problem-01:

If the subnet mask 255.255.255.128 belongs to class C, find-

1. Number of subnets
2. Number of hosts in each subnet

## Solution-

= 255.255.255.128

= 11111111.11111111.11111111.10000000

Since 25 bits contain the value 1 and 7 bits contain the value 0, so-

• Number of Net ID bits + Number of Subnet ID bits = 25
• Number of Host ID bits = 7

Now,

• It is given that subnet mask belongs to class C.
• So, Number of Net ID bits = 24.

Substituting in the above equation, we get-

Number of Subnet ID bits

= 25 – 24

= 1

Thus,

 Number of subnets = 21 = 2

Since number of Host ID bits = 7, so-

 Number of hosts per subnet = 27 – 2 = 126

## Problem-02:

If a class B network has a subnet mask of 255.255.248.0, what is the maximum number of hosts per subnet?

1. 1022
2. 1023
3. 2046
4. 2047

## Solution-

= 255.255.248.0

= 11111111.11111111.11111000.00000000

Since 21 bits contain the value 1 and 11 bits contain the value 0, so-

• Number of Net ID bits + Number of Subnet ID bits = 21
• Number of Host ID bits = 11

Since number of Host ID bits = 11, so-

 Number of hosts per subnet = 211 – 2 = 2046

Thus, Option (C) is correct.

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