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Packet filtering is the process of filtering incoming and /or outgoing data packets on an network interface. Incoming traffic filtering passes or blocks packets that enter the network, while outgoing traffic filtering controls the traffic that leaves the network. A packet filtering firewall may be configured to also inspect outgoing traffic for a company network in order to limit what an attacker can do once they've compromised  the network.
Packet filtering firewalls can be configured to examine incoming traffic as well as outgoing traffic for a company network. This is to ensure the security of the network. 

Even though the highest firewall is in place in a network, there is still a chance that an unwanted intrusion can pass through undetected. In this case, in the event that an unwanted packet was able to pass through incoming traffic, the additional filter in examining outgoing traffic may limit or quarantine the unwanted packet in the area it has already infected. Thus, keeping other parts of the network safe from unwanted intrusion.