Which of the following layers of the ISO/OSI model do packet filtering firewalls operate at?

A.
Application layer

B.
Session layer

C.
Network layer

D.
Presentation layer

Explanation:
Packet filtering firewalls work at the network level of the OSI model, or the IP layer of TCP/IP. These
firewalls are normally part of a router, which is a device that receives and forwards packets to
networks. “In a packet filtering firewall each packet is compared to a set of criteria before it is
forwarded. Depending on the packet and the criteria, the firewall can drop the packet, forward it, or
send a message to the originator.” The criteria used to evaluate a packet include source, destination
IP address, destination port, and protocol used. These types of firewalls are low in cost and don’t
have much of an impact on the network’s performance.