A firewall in the form of security software installed on a proxy server to act as a barrier between internal and external networks and, thereby, to both prevent unauthorized entities from gaining access to internal company resources and block internal users from gaining access to unauthorized external resources. A proxy firewall presents a single network address to the Internet, rather than exposing the true addresses of internal users. Network address translation (NAT) software is required to make the address translations in order that authorized communications can take place in a conversational manner. See also firewall, NAT, proxy server, and security.