They can be easily implemented in either software or in hardware and sometimes are implemented in a combination of the two. All private messages in a certain intranet must go through a firewall. In turn the firewall filters the message and will block alls those messages that fail to meet the security criteria.
A firewall in a Cisco device basically combines the already existing the Context-based Access Control (CBAC) feature with the Cisco IOS firewall technology. So therefore whenever you configure any Cisco Firewall in your Cisco router, you are effectively transforming your router into a robust firewall.
The features of the Cisco firewall are designed specifically to prohibit all unauthorized external persons from getting access to an internal network and to stem all attacks on the network; at the same time it allows all the authorized users to fully access the network resources