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A subnet mask is a 32-bit string that shows where the network portion of an IP address ends and the hosts portion begins. It is important because it tells everything that looks at it how the network is defined, and combined with the IP address shows where a given packet came from. A typical subnet mask might look like this:

11111111.11111111.11111111.00000000

Which in decimal is 255.255.255.0. The area where the subnet mask is 1 means that that portion of the address defines the network, and where the mask is 0, that part of the address defines the individual host on that network.

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