One of the most important skills a network administrator needs is mastery of access control lists (ACLs). ACLs provide security for a network.
Network designers use firewalls to protect networks from unauthorized use. Firewalls are hardware or software solutions that enforce network security policies. Consider a lock on a door to a room inside a building. The lock allows only authorized users with a key or access card to pass through the door. Similarly, a firewall filters unauthorized or potentially dangerous packets from entering the network.