Sitemap Firewall software acts as a gateway. It usually consists of a set of routers between the machine and network. The firewall is of four different types. Application gateway, packet filter gateway, circuit level gateway and proxy server. The most basic firewall is the packet filter firewall. A packet filter firewall filters packets coming into and going out of the machine based on user configuration. If a certain packet does not fit into previously defined user configuration then it is discarded. For example the packet contains some information about the packet. So if the user has blocked a certain port number or IP address all packets coming from that port number or IP address will be discarded. A circuit level firewall does security checkup when the TCP or UDP connection is being set up between host and network. No further checking is done after connection setup. An application level firewall works at the application level. It is used for filtering required by higher level protocols like telnet, FTP, HTTP etc. It is more reliable and efficient than packet filter firewall. Application level gateway helps check mails as well and keeps unwanted or harmful date out of the system by preventing its entry.