Subnet Mask - Subnetting

   The subnet mask is used to divide a network into a further sub network within A private network. Subnetting can be used for security purpose in many cases it enable a network administrator to divide a college network for example into many departments like Botany, Art and Science group network.

   Subnetting is applied on host id we reserve host id bits for subnetting from left to right following example depict all aspects of operation 

What is Subnetting?

Subnetting enables the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format.

The full address is:

10010110.11010111.00010001.00001001

The Class B network part is:
10010110.11010111

The host address is:
00010001.00001001

If this network is divided into 14 subnets, however, then the first 4 bits of the host address (0001) are reserved for identifying the subnet.

The subnet mask is the network address plus the bits reserved for identifying the subnetwork -- by convention, the bits for the network address are all set to 1, though it would also work if the bits were set exactly as in the network address. In this case, therefore, the subnet mask would be 11111111.11111111.11110000.00000000. It's called a mask because it can be used to identify the subnet to which an IP address belongs by performing a bitwise AND operation on the mask and the IP address.
The result is the subnetwork address:
Subnet Mask 255.255.240.000 11111111.11111111.11110000.00000000
IP Address 150.215.017.009 10010110.11010111.00010001.00001001
Subnet Address 150.215.016.000 10010110.11010111.00010000.00000000
The subnet address, therefore, is 150.215.016.000