1. Packet Filtering Firewalls

These firewalls work at the network layer of OSI model, or IP layer of TCP/IP. They are usually part of a router. A router is a device that receives packets from one network and forwards them to another network. In a packet filtering firewall, each packet is compared to a set of criteria before it is forwarded. Depending on the packet and the criteria, the firewall can drop the packet, forward it or send a message to the originator. Rules can include source and destination IP addresses, source and destination port number and type of the protocol embedded in that packet. These firewalls often contain an ACL (Access Control List) to restrict who gains access to which computers and networks.