<human>:  Explain the function of a firewall in a computer system
<bot>: A firewall is a security system designed to prevent unauthorized access to or from a private network. It acts as a barrier between a trusted, secure internal network and other external networks, such as the Internet. A firewall typically establishes a boundary between the internal network and external network through packet filtering, which looks at incoming and outgoing packets of data and either allows or denies access based on a predetermined set of security rules. Firewalls are used to protect computer systems from malicious software, such as Viruses and Trojans, as well as to protect the network from unauthorized access.