What Is A Subnet Mask?
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Subnet Mask, A mask used to determine what subnets an IP address belongs to. An IP address has two components, the networks 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. Subnetting enables the network administrator to further divide the host part of the addr. into two or more subnets. In this case, a part of the host addr is reserved to identify the particular subnet. This is easier to see if we show the IP addr. in binary format. The full addr. is: 10010110.11010111.00010001.00001001 The Class B networks part is: 10010110.11010111 and the host addr. is 00010001.00001001 If this network is divided into 14 subnets, however, then the first 4 bits of the host addr. (0001) are reserved for identifying the subnet. The subnets mask is the network addr. 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 addr.) In this case, therefore, the subnets mask would be 11111111.11111111.11110000.00000000. It's called a mask because it can be used to identify the subnets to which an IP addr. belongs by performing a bitwise AND operation on the mask and the IP addr. The result is the subnetwork addr: |
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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 |
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The subnet address, therefore, is 150.215.016.000. |
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