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subnet mask
Last modified: Friday, November 02, 2001 

A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address 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 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:


The Class B network part is:


and the host address is


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 Mask255.255.240.000 11111111.11111111.11110000.00000000
IP Address150.215.017.009 10010110.11010111.00010001.00001001
Subnet Address150.215.016.000 10010110.11010111.00010000.00000000

The subnet address, therefore, is

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For pages about subnet mask . Also check out the following links!

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An educational course on addressing TCP/IP Networks that includes IP Addresses and Subnetting. Topics include: Binary Math, IP Addressing (IP Address), Subnet Mask, and Custom Subnet Mask.

TCP/IP subnet calculator
Calculates subnet masks, lists the subnets, and figures out the node and network components of a TCP/IP address.

Variable Length Subnet Masking (VLSM)
A very good explanation of what happens when a company divides their network into subnets using variable length masking.

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