Final answer:
Firewalls are used to protect against unauthorized external users by acting as a barrier between a private network and the public internet.
Step-by-step explanation:
Firewalls are used to protect against unauthorized external users. They act as a barrier between a private network and the public internet, monitoring and controlling incoming and outgoing network traffic. Firewalls can be hardware or software-based, and they use various techniques such as packet filtering, stateful inspection, and application-level gateways to prevent unauthorized access to a network.