What is the primary function of a firewall in network security?

The primary function of a firewall in network security is to filter incoming and outgoing network traffic. Firewalls act as barriers that enforce a set of security rules designed to control what traffic is allowed to enter or exit a network. By examining data packets and their headers, firewalls can determine whether the traffic should be allowed based on defined security policies. This filtering is crucial for protecting networks from unauthorized access, cyber attacks, and various types of malicious activities.

Unlike the other options, which refer to functions that do not directly relate to network security, filtering traffic is fundamental to a firewall's role in safeguarding network integrity. Data storage and running applications do not pertain to a firewall's primary purpose, and while monitoring performance can be an aspect of overall network management, it is not the core functionality of a firewall. Thus, filtering traffic is essential for preventing threats and ensuring that only legitimate communication is permitted.