A$-$You need to determine the subnets created by the $20-$bit subnet mask applied to the $172\mathrm{.}16\mathrm{.}0\mathrm{.}0/16$ Network.

Steps:

$1.$ Examining the subnet Mask

The subnet mask can be written using any of the three notations:

Binary notation: $11111111\mathrm{.}11111111\mathrm{.}11110000\mathrm{.}00000000$ Dotted decimal notation: $255.255.240.0$

Classless Inter$-$Domain Routing $($CIDR$)$: merely writes $/20$

Which is the octet of interest for the $20-$bit subnet mask $11111111\mathrm{.}11111111\mathrm{.}11110000\mathrm{.}00000000?$

VLAN B

$2$

The $1$st octet is all $1$s$,$ the $2$nd octet is all $1$s$,$ and the $3$rd octet $41$s and $40$s$.$ Therefore, the $3$rd octet is

Another way to determine this is by looking at the first decimal number which is $172$ class B address therefore the next octet is the $3$rd one to be used for sub$-$ netting.

$2.$ The decimal number of the $3$rd octet in the subnet mask is $240($or $11110000).$ Therefore the address block size is $255-240=16.$ That means there will be $16$ subnets in total $($remember we don$$t exclude the $0000$ and $1111$ combinations of the borrowed for sub$-$netting bits$-$ The number of subnets $=24=16)$

$3.$ The first subnet is $172\mathrm{.}16\mathrm{.}0\mathrm{.}0(172\mathrm{.}16\mathrm{.}0\mathrm{.}0/20$ with the $4$ borrowed bits in the $3$rd octet set to $0)$

$4.$ Beginning with the first subnet of $172\mathrm{.}16\mathrm{.}0\mathrm{.}0/20$ and counting the block size of $16$ in the interesting octet yields

the following subnets:

# $3$rd octet

$00000-0000$

$10001-0000$

$20010-0000$

$30011-0000$

$40100-0000$

$50101-0000$

$60110-0000$

$70111-0000172\mathrm{.}16.112.0/20$

$81000-0000$

$91001-0000$

$101010-0000$

$172\mathrm{.}16.128.0/20$

$172\mathrm{.}16.144.0/20$

$172\mathrm{.}16.160.0/20$

Subnet$/$Mask

$172\mathrm{.}16.0.0/20$

$172\mathrm{.}16.16.0/20$

$4-$bits borrowed for sub$-$netting

$172\mathrm{.}16.32.0/20$

$172\mathrm{.}16.48.0/20$

$172\mathrm{.}16.64.0/20$

$172\mathrm{.}16.80.0/20$Subnet for VLAN A $172\mathrm{.}16.96.0/20$

$111011-0000172\mathrm{.}16.176.0/20$

$121100-0000172\mathrm{.}16.192.0/20$

$131101-0000172\mathrm{.}16.208.0/20$Subnet for VLAN B $141110-0000172\mathrm{.}16.224.0/20$

$151111-0000172\mathrm{.}16.240.0/20$

$3$rd octet