Subnet Mask | How to Calculate Subnet Mask

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-

 

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?

 

For any given IP Address, the subnet mask is calculated-

• 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.

 

Subnet Mask Examples-

 

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.

 

Subnet mask is obtained-

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

 

So, Subnet mask

= 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.

 

So, Subnet mask

= 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.

 

So, Subnet mask

= 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.

 

So, Subnet mask

= 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.

 

Use of Subnet Mask-

 

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

 

Important Notes-

 

Note-01:

 

Default mask for different classes of IP Address are-

• 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

 

Also Read- Classes of IP Address

 

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).

 

PRACTICE PROBLEMS BASED ON SUBNET MASK-

 

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-

 

Given subnet mask

= 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-

 

Given subnet mask

= 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.

 

To gain better understanding about Subnet Mask,

Watch this Video Lecture

 

Next Article- Practice Problems On Subnet Mask

 

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