Network Appliance for Packet Legitimacy Check

D.

The network appliance that checks the state of a packet to determine whether the packet is legitimate is a firewall.

A firewall is a security device that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It acts as a barrier between a trusted internal network and an untrusted external network, such as the internet.

Firewalls use various methods to determine whether a packet is legitimate, including packet filtering, stateful inspection, and application-level gateways. Packet filtering is the most basic form of firewall protection, which filters packets based on source and destination IP addresses, ports, and protocols. Stateful inspection is a more advanced form of packet filtering that examines the state of a connection to determine whether a packet is part of a legitimate session. Application-level gateways, also known as proxies, go one step further by examining the contents of packets to ensure that the data is safe and appropriate for the application.

In summary, a firewall is a network appliance that checks the state of a packet to determine whether the packet is legitimate. It does this by using various security techniques, including packet filtering, stateful inspection, and application-level gateways.