Define the subnet mask
The number of bits used for the subnet mask depends on the number of possible subnets and the number of hosts per subnet. Before defining the subnet mask, you must figure out the number of subnets and hosts that were originally used.
The steps to define the subnet mask are:
A. Determine which group addresses are used by us. For example, the network number we applied for is "210.73.ab", the network address is a class c IP address, the network ID is "210.73.a", and the host ID is "b".
B. According to the number of subnets we need and the number of subnets that may be expanded in the future, use some bits of the host to define the subnet mask. Say we need 12 subnets, maybe 16 in the future. Use the first four bits of the fourth byte to determine the subnet mask. The first four bits are all set to "1", that is, the fourth byte is "11110000", which we call the new binary subnet mask for now.
C. Set each bit corresponding to the initial network to "1", that is, the first three bytes are all set to "1", then the intermittent binary form of the subnet mask is: "11111111.11111111.11111111.11110000".
D. Convert this number to discontinuous decimal form: "255.255.255.240".
Reference link:
subnet mask
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