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Subnet Mask

A subnet mask is a number that defines a range of IP addresses that can be used in a network. (It is not something you wear on your head to keep subnets out.) Subnet masks are used to designate subnetworks, or subnets, which are typically local networks LANs that are connected to the Internet. Systems within the same subnet can communicate directly with each other, while systems on different subnets must communicate through a router. Therefore, subnetworks can be used to partition multiple networks and limit the traffic between them.

A subnet mask hides, or "masks," the network part of a system's IP address and leaves only the host part as the machine identifier. A common subnet mask for a Class C IP address is Each section of the subnet mask can contain a number from 0 to 255, just like an IP address. Therefore, in the example above, the first three sections are full, meaning the IP addresses of computers within the subnet mask must be identical in the first three sections. The last section of each computer's IP address can be anything from 0 to 255. For example, the IP addresses and would be in the same subnet, while would not. Therefore, a subnet mask of allows for close to 256 unique hosts within the network (since not all 256 IP addresses can be used).

If your system is connected to a network, you can typically view the network's subnet mask number in the Network control panel (Windows) or System Preference (Mac OS X). Most home networks use the default subnet mask of However, some office networks may use a different subnet mask such as, which can be used to split a network into two subnets. Large networks with several thousand machines may use a subnet mask of This is the default subnet mask used by Class B networks. The largest Class A networks use a default subnet mask of

Published: 2008

Definition from the PC Glossary