What components are commonly found in a firewall?

What will be an ideal response?

A firewall can contain many components, including a packet filter, a proxy server, an authentication system, and software that perform Network or Port Address Translation (NAT or PAT). Some firewalls can encrypt traffic, and some help establish VPNs. Some firewalls are packaged in a hardware device that also functions as a router. Firewalls themselves are often part of multiple-component security setups. The most effective protection systems used by large corporate networks employ not just one but several firewalls. They combine the firewalls with routers and other components to delineate zones of trust such as a screened subnet, also known as a demilitarized zone (DMZ), which is positioned between the internal network and the outside world.