3. Application gateways Firewall

 These are the software firewalls. These are often used by companies specifically to monitor and log employee activity and by private citizens to protect a home computer from hackers, spy ware to set parental controls for children.

Application gateways also called proxies are similar to circuit level gateways expect that they are application specific. They can filter packets at the application layer of OSI or TCP/IP model. Incoming or outgoing packets can’t access services for which there is no proxy. In plain terms, an application level gateway is configured to be a web proxy will not allow all ftp, gopher, telnet or other traffic through. Because they examine packets at the application layer, they can filter application specific commands such as http: post, get etc;

It works like a proxy. A proxy is a process that sits between a client and a server. For a client proxy looks like a server and for a server, the proxy looks like a client.Example Application layer firewall: In an application layer firewall called a ``dual homed gateway'' is represented. A dual homed gateway is a highly secured host that runs proxy software. It has two network interfaces, one on each network, and blocks all traffic passing through it.