It interconnects two or more cabling-system segments. The harmful thing is the volume of frames on each of these cabling-system segments. Otherwise, the frame is transmitted out the port that leads to the destination device. However, multicast frames must be transmitted out all other ports. A switch is a firewall .
Some switches only permit a single device to be connected to a port.
Others permit either a single device or a cabling-system segment to be connected to a port. A router is a firewall . It interconnects two or more otherwise-unconnected transmission systems. The harmful thing is the volume of frames on the transmission system. A router with access-control rules is a firewall .