With a firewall between your network and the Internet, all traffic coming from the Internet must pass through the firewall before entering your network. Firewalls can also be used to control access to a specific part of your network. For example, you can position firewalls at all the entry points into a research and development network to prevent unauthorized access to proprietary information. The most basic function of a firewall is to monitor and filter traffic. Firewalls can be simple or elaborate, depending on your network requirements.
Simple firewalls are usually easier to configure and manage.
However, you might require the flexibility of a more elaborate firewall. The Cisco IOS Firewall Solution Cisco IOS software provides an extensive set of security features, allowing you to configure a simple or elaborate firewall, according to your particular requirements. You can configure a Cisco device as a firewall if the device is positioned appropriately at a network entry point. Security features that provide firewall functionality create a customized Firewall. In addition to the security features available in standard Cisco IOS feature sets, there is a Cisco IOS Firewall feature set that gives your router additional firewall capabilities.