Firewall

From Wikipedia:

A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules.

The first reported type of network firewall is called a packet filter. Packet filters act by inspecting packets transferred between computers. When a packet does not match the packet filter's set of filtering rules, the packet filter either drops (silently discards) the packet, or rejects the packet (discards it and generates an Internet Control Message Protocol notification for the sender) else it is allowed to pass. Packets may be filtered by source and destination network addresses, protocol, source and destination port numbers. (Similar to a router, but filtering, not routing).