Methods and systems for an intelligent network protection gateway (NPG) and network architecture are provided. According to one embodiment, a firewall provides network-layer protection to internal hosts against unauthorized access by hosts of an external network by performing network address translation (NAT) processing of Internet Protocol (IP) addresses. The firewall changes data in headers of VoIP packets and corresponding data contents of the VoIP packets, to enable bi-directional VoIP communications. An external VoIP interface of the firewall receives incoming VoIP packets having a user alias (e.g., an email address) and an indication regarding a VoIP port of external interface. The packets are directed to an appropriate internal host by the firewall performing port address forwarding based on the port indication to a Session Initiation Protocol (SIP) server within the internal network that maintains a mapping of user aliases to private addresses of the internal hosts.