a) [3 marks] What is a firewall and what does it do? b) [6 marks] There are several different major types of firewall that operate at different levels. Describe the differences between packet-level firewalls, application level firewalls and NAT firewalls. And what does NAT stand for? c) [6 marks] What is IPSec? At which levels in the protocol stack does IPSec operate? Describe how IPSec works