The purpose of a firewall in a network is to serve as a security system that monitors and controls incoming and outgoing network traffic. By establishing a barrier between a trusted internal network and untrusted external sources, firewalls help protect sensitive data from unauthorized access, malware, and other cyber threats. Firewalls operate using predefined rules to allow or block specific traffic, enhancing the overall security posture of the network. They can be implemented through hardware, software, or a combination of both, ensuring robust protection for networked environments.