Subnetting concept and Masking

Question

How many subnets & hosts per subnet are possible if m bits are borrowed from host ID (HID).
a) 2^m,2^(HID-m)-2

b)2^m,2^(HID-m)

c)2^m-2,2^(HID-m)-2

d)2^m-2,2^(HID-m).

The number of subnets will be 2^m but confused with number of hosts per subnet why.

Answer 1

A) subnets are 2^m & hosts per subnet are 2^hid-m - 2.

Comments

Can you pleasr explain the second part about -2
Some places don't need to leave the first and last address while some do.

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We should left the first & last address in each network... Because after subnetting each network will be an independent entity... So for any network, first & last addresses are network address(all 0s in host part) and broadcast address(all 1s in host part) respectively....

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Consider this
192.168.240.0/24
now
you are using 192.168.240.0 and 192.168.240.255 for two purpose ,

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Yes you are correct... First one is network address & the second one acts as a broadcast address.... 

For more understanding, https://youtu.be/87TiQ6mIy9w