The following describes a personal firewall that is intended for home or small business use. It also lists some of the different types of firewalls, and some issues that you may experience when you use a firewall.

Different firewalls use different techniques. Most firewalls use two or more of the following techniques:

• Packet filters: A packet filter looks at each packet that enters or leaves the network and accepts or rejects the packet based on user-defined rules. Packet filtering is fairly effective and transparent, but it is difficult to configure. In addition, it is susceptible to IP spoofing.
• Application gateway: An application gateway applies security mechanisms to specific programs, such as FTP and Telnet. This technique is very effective, but can cause performance degradation.
• Circuit-layer gateway: This technique applies security mechanisms when a Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) connection is established. After the connection has been established, packets can flow between the hosts without further checking.
• Proxy server: A proxy server intercepts all messages that enter and leave the network. The proxy server effectively hides the true network addresses.
• Application proxies: Application proxies have access to the whole range of information in the network stack. This permits the proxies to make decisions based on basic authorization (the source, the destination, and the protocol), and also to filter offensive or disallowed commands in the data stream. Application proxies are "stateful," meaning that they keep the "state" of connections inherently. The Internet Connection Firewall feature that is included in Windows XP is a "stateful" firewall, as well as Windows Firewall. Windows Firewall is included in Windows XP Service Pack 2 (SP2).