$($a$)$ Give the subnet mask for dividing the Network $20\mathrm{.}0\mathrm{.}0\mathrm{.}0$ into $30$ equal$-$size subnets.

$(5$ marks$)$

$($b$)$ Continue from question $3($a$),$ give the subnet address for $20\mathrm{.}158\mathrm{.}100\mathrm{.}50.$

$(5$ marks$)$

$($c$)$ Continue from question $3($a$),$ provide the feasible IP address range in the $20$th subnet.

$(6$ marks$)$

$($d$)$ In the information technology area, firewall can be used for the protection against network

attacks. Briefly describe what the purposes of packet filtering are.

$(4$ marks$)$