How can a firewall be used to restrict access to a computer network?

A firewall can restrict access to a computer network by monitoring and controlling incoming and outgoing network traffic based on predetermined security rules. It can block unauthorized access attempts from external sources while allowing legitimate traffic through. By setting specific rules, such as allowing only certain IP addresses or protocols, a firewall helps protect the network from threats and unauthorized users. Additionally, firewalls can provide logging and alerting features to notify administrators of suspicious activities.