An apparatus and method for providing a secure firewall between a private network and a public network are disclosed. The apparatus is a gateway station having an operating system that is modified to disable communications packet forwarding, and further modified to process any communications packet having a network encapsulation address which matches the device address of the gateway station. The method includes enabling the gateway station to transparently initiate a first communications session with a client on a first network requesting a network service from a host on a second network, and a second independent communications session with the network host to which the client request was addressed. The data portion of communications packets from the first session are passed to the second session, and vice versa, by application level proxies which are passed the communications packets by the modified operating system. Data sensitivity screening is preferably performed on the data to ensure security. Only communications enabled by a security administrator are permitted. The advantage is a transparent firewall with application level security and data screening capability.