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I have lately taking interest in networking so I picked up a book to get acquainted with networking principles. The book I have chosen is that of Glen E. Clarke "CompTIA Network+ ...." which is supposed to be a preparation book for the Network+ certification. On the subnetting chapter of it, as an example, we wanted to divide the following (Class A)address: 10.0.0.0 into 2 subnetworks.

On this example, he used the following formula: (2^number_of_masked_bits) - 2 = number of networks.

using this he created this table with the note above it.

On this exact same book but on a different edition, where he (this time) tried to divide the network into FOUR sub-networks, he used the following formula: (2^masked_bits) = number of networks. And he took the table as is and never declared any address to be illegal !!!:

enter image description here

Can you please explain to me how does this work ? thank you

Achraf Badiry
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    The linked question and answers will explain it all for you. – Ron Trunk Mar 27 '24 at 11:32
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    If anything you read talks about network classes or VSLM, please remember that this is completely obsolete, and has been since before you were born. For some inexplicable reason, it is still taught. – Ron Trunk Mar 27 '24 at 11:37
  • Yes, VLSM-based subnetting has been obsoleted in 1993, more than 30 years ago. It's all CIDR since (and much simpler), see the excellent Q&A linked above your question. – Zac67 Mar 27 '24 at 11:46
  • Thank you very much :) – Achraf Badiry Mar 27 '24 at 13:17

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