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Glossary: Subnet Masking for home networks

The subnet mask of a network tells network devices such as IP cameras, computers and printers how to understand the IP address it has been given and how it should communicate with other network devices. The subnet mask is split into two sections. The first specifies the network portion of an IP address and the second specifies the device address.

A typical subnet mask is 255.255.255.0. Whenever an IP address is given to a device it compares each number block against the subnet mask. In this example the first three number blocks (255) are the network address and the last (0) is the device address. Since each number block in an IP address can be between 1 and 254 (0 and 255 are reserved) you can have 254 different devices.

Another common subnet mask is 255.255.0.0. This means the first two number blocks of an IP address are the network address while the second two are the device address. Those of you who are numerically minded will know that this allows for 64,516 different devices on the same network.

It does however get tricky when you start looking at subnet masks such as 255.255.255.248 which only allows 6 seperate devices to be used. My advice would be that if you are using less than 250 devices on your network, stick with a subnet of 255.255.255.0.

The subnets of each device on a network have to be the same in order for them to share information. Any information which does not share same network address ( i.e. in the earlier example 192.168.0 ) will be sent to the default gateway rather than the local network.

Published on July 22nd, 2008 by James Drinkwater

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