A computer system contains multiple Internet communications stack instances, which may or may not share a common hardware network adapter. Packets are routed between different Internet communications stack instances internally within the computer system using Internet Protocol (IP) addressing. A packet arriving in one stack and having a destination IP address associated with another stack is forwarded to the other stack using IP forwarding. Preferably, inter-stack routing of packets may use either globally defined Internet IP addresses or local intranet (encapsulated) IP addresses, and may apply to either inbound or outbound packets. An exemplary embodiment is a production stack having a full range of TCP/IP functions, and a service stack having a limited range of TCP/IP functions. The inter-stack interface can be used to obtain advanced function operations for packets arriving for and being sent by applications bound to the service stack.