The simplest form of a firewall is a packet filtering firewall. Typically a router works as a packetfiltering firewall and has the capability to filter on some of the contents of packets. On which of the
following layers of the OSI reference model do these routers filter information? Each correct answer
represents a complete solution. Choose all that apply.

A.
Transport layer

B.
Physical layer

C.
Data Link layer

D.
Network layer

Explanation:
Typically routers work as packet-filtering firewalls. These routers have the capability to
filter on some of the contents of packets. The
information that a packet filtering firewall can examine includes the Network layer (layer 3) and
sometimes the Transport layer (layer 4)
information. For example, Cisco routers with standard ACLs filter information at the Network layer.
However, Cisco routers with extended ACLs
filter information at both the Network layer and Transport layer.
Answer option B is incorrect. The Physical Layer defines the means of transmitting raw bits rather
than logical data packets over a physical link
connecting network nodes.