A Packet Level Firewall protects an internal network against unauthorized access and attack from a public or external network by blocking some packets. Packets may be blocked based on source IP address, destination IP address, source or destination TCP port number, other packet header fields, time of day or user authentication.
Filtering is carried out on a packet-by-packet basis, and the
entire Packet Level Firewall may be implemented on a Router. Normally all
filtering happens inside the Operating System, which makes
this type of Firewall very fast.