Firewall port/protocol rules are used to block or allow network packets to pass through the firewall. This process starts by examining control information contained within each individual packet. The rules then determine if the packet can be safely allowed through to the network.
Firewall rules are assigned specifically to computers, or can be applied in the form of policies that are assigned to a collection of computers. A firewall can use an IP address, MAC address, or a port to establish the source of traffic and its destination.