Filtering Firewalls - that block selected network packets. Packet Filtering Firewalls Packet Filtering is the type of firewall built into the Linux kernel. A filtering firewall works at the network level. Data is only allowed to leave the system if the firewall rules allow it. As packets arrive they are filtered by their type, source address, destination address, and port information contained in each packet.
Many network routers have the ability to perform some firewall services.
Filtering firewalls can be thought of as a type of router. Because of this you need a deep understanding of IP packet structure to work with one. Because very little data is analyzed and logged, filtering firewalls take less CPU and create less latency in your network. Filtering firewalls do not provide for password controls. User can not identify themselves.