troubleshooting Question

Subnetting questions - CCNA related

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Dilan77 asked on
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I've got a question on subnetting that hopefully you'll be able to help me on....
Given a network address and a subnet mask, how can you work out the number of subnets created. Is it
2 to the power of borrowed bits
(2 to the power of borrowed bits) - 2
So, for example
Network address:
Is the number of subnets created 2 (to the power 2) = 4
Or 2 (to the power 2) - 2 = 2
Some sources say the first and last subnet created are not valid, hence the reason for subtracting two. Obviously, for hosts, the first and last hosts are invalid because they represent the network and broadcast address, but I thought routers could now handle the zero subnet by default? Also, why would the last subnet be invalid? What is/was the reasoning behind this.

Would appreciate the help.

Pete Long
Solutions Architect
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