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At the higher end are the proxy-server gateways that perform proxy services for internal clients by regulating incoming external network traffic and by monitoring and providing traffic control of outgoing internal packets. Packet-filtering firewalls are routers that operate in the low levels of a network protocol stack. The most basic type of firewall is a packet-filtering device, also known as a screening router.
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The three major types of firewalls utilize different methods to basically accomplish the same thing-protect an internal network. There are three major types of firewalls used for protecting an enterprise's Intranet, but any device that controls traffic flowing through a network for security reasons can be considered a firewall. The main function of a firewall is to protect the internal proprietary data from the outside world. Tim Speed, Juanita Ellis, in Internet Security, 2003 5.1.2 Assessing the right type of firewall(s) for your enterprise Today, this type of firewall is considered very basic and limited, and may even be included in operating systems as an “extra.” Lastly, the static packet filter is not state-aware, so the administrator is required to configure rules for both sides of the conversation. Another inherent limitation is that the static packet filter does not examine the entire packet, which makes it possible for an attacker to hide malicious commands inside unexamined headers or within the payload itself. Another limitation is that for larger installations, the static packet filter becomes unwieldy because packet-filtering rules are examined in sequential order, and care must be taken when entering rules into the rule base. Also, because it examines only the packet headers, attackers can bypass the static packet filter with simple spoofing techniques, since the filter cannot tell the difference between a true and a forged address. The packet filter may lack logging facilities, which would make it impractical for an organization that has compliance and reporting requirements to which they must adhere. The packet filtering firewall filters IP packets based on source and destination IP address, and source and destination port. In addition, faster networks are more capable of handling the greater processing requirements of a firewall that operates at a higher level of the OSI stack.
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However, today's higher-level firewalls deliver excellent performance as well. The static packet filter does not impact performance to any noticeable degree, and its low processing requirements made this an attractive option early on when compared to other firewalls that dragged down responsiveness. This type of firewall decides whether to accept or deny individual packets, based on examining fields in the packet's IP and protocol headers. The static packet filtering firewall operates only at the network layer (layer 3) of the OSI model and does not differentiate between application protocols. Packet filtering firewalls are among the oldest firewall architectures. Jack Wiles, in Techno Security's Guide to Securing SCADA, 2008 Static Packet Filter