A firewall is a network security system that monitors and controls incoming and outgoing traffic based on predefined security rules.

Elaboration:

• Operates at Layer 3 (Network Layer) and Layer 4 (Transport Layer) of the OSI Model.
• Can be hardware-based (physical firewalls) or software-based (Windows Firewall, iptables).
• Implements security features like packet filtering, intrusion detection, and access control lists (ACLs).

Example:

A company’s firewall blocks unauthorized access to its internal network while allowing legitimate traffic.