On a fundamental level, the purpose of a network firewall is to act as a packet filter. A firewall is placed on the boundary between trusted (internal) networks and untrusted (external) networks. When packets coming from the external network to the internal network do not match the packet filter’s set of rules (ruleset), the packets are either silently discarded (in other words, dropped) or an Internet Control Message Protocol (ICMP) message is returned to the sender (in which case we say the packet was rejected). If the packets match the ruleset, they are allowed through the firewall (in which case we say the packet passes through).
As you might have deduced, a good default firewall policy is to block all traffic not ...
