given class A

$122\mathrm{.}0\mathrm{.}0\mathrm{.}0$

we need $8100$ subnets

so we need ceil$($log$2(8100))$ bits to represent the $8150$ subnets

ceil$(12\mathrm{.}9837061927)=13$

so we need $13$ bits for subnet

b$)$

subnet mask $=1\text{'}$s for $8+13=21$ bits and rest all $0\text{'}$s

$11111111\mathrm{.}11111111\mathrm{.}11111000\mathrm{.}00000000$

$=255\mathrm{.}255\mathrm{.}248\mathrm{.}0$

c$)2112=20482=2046$

d$)$

Subnet $7851=1111010101011$

Host number $=1964=11110101100$

$01111010111101010101111110101100$

$=>122\mathrm{.}245\mathrm{.}95\mathrm{.}172$