A packet-level filtering firewall

intercepts incoming packets at the network layer then uses an inspection engine to extract state-related information from upper layers.

is commonly a host computer that is running proxy server software, making it a proxy server.

examines both the source and destination address of the incoming data packet then either blocks the packet or passes it to its intended destination network.

Explanation

The packet-filtering firewall examines both the source and destination address of the incoming data packet. This firewall either blocks the packet or passes it to its intended destination network. The firewall can allow or deny access to specific applications or services based on the Access Control Lists (ACLs). ACLs are database files that reside on the firewall, are maintained by the firewall administrator, and tell the firewall specifically which packets can and cannot be forwarded to certain addresses. The firewall can also be configured to allow access for only authorized application port or service numbers. It looks at the data packet to get information about the source and destination addresses of an incoming packet, the sessionâs communications protocol (TCP, UDP, or ICMP), and the source and destination application port for the desired service. A packet-level firewall doesnât keep a history of the communications session. It operates at the Network Layer of the OSI model and offers good performance. Ongoing maintenance of the ACLs can become an issue. Figure 3-20 shows an external router being used as a simple packet-filtering firewall