A method of guarding a private network by analyzing the data leaving and entering. Firewalls can also provide network address translation, so the IP addresses of computers inside the firewall stay hidden from view. "Packet-filtering firewalls" use rules based on a packets source, destination, port or other basic information to determine whether or not to allow it into the network. More advanced "stateful packet filtering firewalls" have access to more information from which to make their decisions. "Proxy firewalls," which look at content and can involve authentication and encryption, can be more flexible and secure but also tend to be far slower. Although firewalls are difficult to configure correctly, security experts generally agree that they are a critical component of network security.