Final answer:
A packet-filtering firewall examines the source and destination addresses and other data of a message to determine if it should be allowed through the network based on pre-defined rules.
Step-by-step explanation:
A packet-filtering firewall examines the source address, destination address, and other data of a message and determines whether to let that message pass. Packet-filtering firewalls are crucial components in network security and operate at the network layer of the OSI model. They act by inspecting individual packets of data that are transmitted over a network. Based on a set of rules defined by the network administrator, the firewall decides whether to allow or block the passage of these packets. The rules can be based on IP addresses, protocol, port number, and various other attributes. If the data packet meets the set criteria, it's allowed to pass; otherwise, it is blocked. This helps in preventing unauthorized access and can protect against a range of network threats. Among the given options, an encrypted firewall refers broadly to a firewall that applies encryption to enhance security, but it does not specifically describe the function of filtering packets based on rules. Internal malware is not a safeguarding system but refers to malicious software within the network. Perimeter shareware is not a conventional term used in networking and seems out of context for the described function.