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A firewall is a critical component of network security, designed specifically to filter and control incoming and outgoing network traffic based on predetermined security rules. Its primary function is to create a barrier between a trusted internal network and untrusted external networks, such as the internet. By analyzing the data packets that attempt to enter or leave the network, a firewall can block or allow traffic based on the rules set by security administrators. This helps prevent unauthorized access, cyberattacks, and malicious activities while allowing legitimate communications to proceed.

In contrast, encrypting sensitive data focuses on ensuring data confidentiality and integrity during transmission or storage, which is different from the traffic control functions of a firewall. Performing vulnerability assessments involves identifying weaknesses in a system or network rather than managing the flow of data. Monitoring employee internet usage relates to oversight and compliance rather than security per se; while it can contribute to overall security posture, it does not encompass the core purpose of a firewall.

Thus, the function of filtering and controlling network traffic is fundamental to protecting a network from threats, making it the correct answer regarding the role of a firewall in network security.