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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 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 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 and 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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 LearnToSubnet.Com
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.
Variable Length Subnet Masking (VLSM)
A very good explanation of what happens when a company divides their network into subnets using variable length masking.
NetworkCLue: Calculating Subnet Masks
The easiest way to explain a subnet mask is by looking at the IP address and subnet mask in its binary format.
PracticallyNetworked.com
PracticallyNetworked.com provides easy-to-understand help for small-network builders. The site contains how-to information for setting up and debugging home-office and small-business networks. Users can also find extensive troubleshooting information, tips on getting applications to work through firewalls, product reviews on network hardware and software, and more.
TCP/IP subnet calculator
Calculates subnet masks, lists the subnets, and figures out the node and network components of a TCP/IP address.
Webopedia's Quick Reference Section
Use this Webopedia reference section for information on common Internet and computer facts and occurrences.
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