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.