In addition, the security solution should also provide detailed log and audit information containing a history of all network communications by user. This element provides the means to control access to critical network applications, data, and services so that only legitimate users and information can pass through the network. Routers and switches with access control lists and stateful firewalling, as well as dedicated firewall appliances, provide this control. Complementary tools, including virus scanners and content filters, also help control network perimeters. Using firewalls you can employ techniques like NAT and the use of a DMZ to protect assets.
Firewalls primarily use one of three technologies to filter data.
Packet filters, historically implemented on routers, filter on user defined content, such as IP addresses. They examine a packet at the network layer and are application independent, which allows them to deliver good performance and scalability. They are the least secure type of firewall, however. The reason is that they are not application aware - that is, they cannot understand the context of a given communication making them easier for hackers to break. Application gateways improve on security by examining all application layers, bringing context information into the decision process.