Determine how many subnets are required and how many hosts per subnet are required.
Use the 2n – 2 formula to determine the number of subnet bits and the number of host bits that will satisfy the requirements established in Step 1. If multiple subnet masks can satisfy the requirements, choose the one that will best scale to future needs. For example, if the network is most likely to grow by adding subnets, choose more subnet bits; if the network is most likely to grow by adding hosts to existing subnets, choose more host bits. Avoid choosing a scheme in which either all subnets or all host addresses within the subnets will be used...
How to subnets
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