An IP address has two components, the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network.
Subnet Mask?
A portion of a network that shares a common address component. On TCP/IP networks, subnets are defined as all devices whose IP addresses
have the same prefix. For example, all devices with IP addresses that
start with 100.100.100. would be part of the same subnet. Dividing a
network into subnets is useful for both security and performance
reasons. IP networks are divided using a subnet mask.
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.
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