Subnetting

Subnetting

It means to split a network or to break a large network into small networks. it is use to stop broadcasting, to manage IP addressing, to save IP addresses etc. It is use for Intranet,  not for Internet.  Number of hosts will be reduce after the subnetting is done.

Subnetting can be done by 2 ways,  CLSM and VLSM.

CLSM (Constant Length Subnet Mask) means every subnet will have same subnet mask and requirement for networks will be given.

VLSM (Variable Length Subnet Mask)  means every subnet will have different subnet mask and requirement for hosts will be given. No IP address will be wasted and we must start from largest network.

NOTE:

To calculate number of networks  2^x

To calculate number of hosts  2ⁿ -2

Formula for subnetting    2ⁿ -2  X

where n = number of host bit and X = number of networks

Example of CLSM :-

We have  150.160.10.20  (150.160.0.0) and we need 5 subnets or networks

Step1  Calculate the minimum value of n, 2ⁿ -2  X

        =>        2ⁿ -2  5

                        8-2  5

                        n = 3                                                        

                                                                               

                                                                               

Step2  Calculate the subnetted mask

NOTE: Subnet Mask tell the number of network bits

          Subnetted Mask increase the number of network bits

Default Subnet Mask =     255  .  255  .  0  .  0

Binary  =>         11111111.11111111.00000000.00000000

                                W       X             Y              Z

                                                

Subnetted Mask = 11111111.11111111. 11100000.00000000

Decimal   =>                     255. 255. 224.0      /19

Step3        Calculate the range of subnets

        NOTE:

        Check value of last network bit in subnetted mask ( 32 )

        Check the octet in which the change has been occur  ( Y )

        150.160.0.0  ( or  150.160.0.1  to 150.160.255.254 )

1. 150.160.0.1            -       150.160.31.254  

2. 150.160.32.1         -       150.160.63.254

3. 150.160.64.1         -       150.160.95.254

4. 150.160.96.1         -       150.160.127.254

5. 150.160.128.1       -       150.160.159.254

6. 150.160.160.1       -       150.160.191.254

7. 150.160.192.1       -       150.160.223.254

8. 150.160.224.1         -       150.160.255.254

NOTE: In every question of subnetting, first and last subnets are  invalid or useless but we can use IP-SUBNET ZERO command in router to utilize first and last subnets. Some OS are manipulated to utilize it.

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172.16.25.35     20 subnets

100.100.10.10     60 subnets

10.120.240.34   1000 subnets

Example of VLSM :-

We have  192.168.25.0  and we need     80 hosts in Mumbai,   50 hosts in London and 30 hosts in Delhi.

Mumbai :-

2ⁿ -2  X     =>   2ⁿ -2 80   => n=7 (host bit)

32 bit - 7  = 25 (network bit) or  /25

192.168.25.1  to 192.168.25.80    /25 ( 255.255.255.128 )

London :-

2ⁿ -2  X 

2ⁿ -2  50

n= 6      32-6    =   /26

192.168.25.81   to 192.168.25.130   /26

Delhi :-

2ⁿ -2  X

2ⁿ -2  30

n = 5

32-5 = /27  192.168.25.131   to  192.168.25.160