The present invention relates to a firewall for use in association with real-time Internet applications such as Voice over Internet Protocol (VoIP). The firewall applies an application proxy to the signaling and control channels and a packet filter to the bearer channels. One of the features of hybrid firewall is that the application proxy can instruct the packet filter as to which bearer channels to enable and disable for the duration of a real-time Internet application session. The hybrid firewall can also intelligently perform network address translation (NAT) on Internet protocol packets incoming and outgoing to the firewall.