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  • Eric Stewart thinks this is interesting:

When you are thinking classfully, if the mask implies x subnet bits, then 2x possible subnets exist for that classful network, assuming that the same mask is used throughout the network.

From

Cover of CCIE Routing and Switching v5.0 Official Cert Guide, Volume 1, Fifth Edition

Note

 When you are thinking classfully, if the mask implies x subnet bits, then 2^x possible subnets exist for that classful network, assuming that the same mask is used throughout the network.