What is Subnetting?

Subnet is a segment of a network. Subnetting is a technique that allows a network administrator to divide one physical network into smaller logical networks and to limit the scope of broadcast traffic, network security issues, and to separate network segments by function.

Let us take as example of how to do one bit, two bit and three bit subnetting. We purchased network id 201.100.50.0. In subnetting we move a host bit into network and make subnets , formula for calculation of subnetwork is 2ⁿ -2 and for host in each subnet is 2ⁿ-2 where as n stands for no bits sifted to network portion for subnetwork and n stands for remaining bits of hosts for number of hosts in a network.

Example

200.100.50.0

Network part 24 bits

Host part 8bits

One bit Subnetting

No of subnets­­­ = 2ⁿ-2   No of host in each subnet = 2ⁿ-2         

                        No of subnets­­­ = 2¹-2 = 2 subnets (n bits shifted to network)

No of host in each subnet = 2⁷-2 =128-2 =126 (n remaining bit in host)

First Subnet                  200.100.50.0

                        {            .      .     .    .

                                      .      .     .    .

                                    200.100.50.127

Second Subnet            

200.100.50.128

                        {            .      .     .    .

                                      .      .     .    .

                                    200.100.50.255

Two Bit Subnetting

No of subnets­­­ = 2ⁿ-2   No of host in each subnet = 2ⁿ-2         

                        No of subnets­­­ = 2²-2 = 4-2=2 subnets (n bits shifted to network)

No of host in each subnet = 2⁶-2 =64-2 =62 (n remaining bit in host)

First Subnet                  200.100.50.0

                        {             .      .     .    .

                                      .      .     .    .

                                    200.100.50.63

Second Subnet             200.100.50.64

                        {             .      .     .    .

                                      .      .     .    .

                                    200.100.50.127

Third Subnet                200.100.50.128

                        {             .      .     .    .

                                      .      .     .    .

                                    200.100.50.191

Fourth Subnet              200.100.50.192

                        {             .      .     .    .

                                      .      .     .    .

                                    200.100.50.255

Now if we again look at the formula we will see 2²-2 = 4-2, here 4 is total no of subnets and -2 means that we will not use first and last subnet. Similarly in hosts 2⁶ -2= 64-2=62, here 64 total no hosts in each subnet and -2 means we will not use first and last address because first address is subnetwork ID and last is broadcast  ID of the subnetwork .Then we will left with 62  usable hosts in each subnet.

Second Subnet             200.100.50.65

                        {             .      .     .    .

                                      .      .     .    .

                                    200.100.50.126

Third Subnet                200.100.50.129

                        {             .      .     .    .

                                      .      .     .    .

                                    200.100.50.190

So finally we will use only second and third subnet and each subnet first and last IP will not be used because they are subnet Ids and Broad cast Ids and you will left with 62 no of usable hosts. Similarly we perform three, four bit etc subnetting.

Note in one bit subnetting I have not done -2 from the network why? Because 2¹-2= 0, it means that we are left with 0 subnet. So remember that in class C one bit subnetting is not possible.