The term dual-stack normally refers to a complete duplication of all levels in the protocol stack from applications to the network layer. One example of complete duplication is a system that runs both the OSI and TCP/IP protocols.
Oracle Solaris is dual-stack, meaning that Oracle Solaris implements both IPv4 and IPv6 protocols. When you install the operating system, you can choose to enable the IPv6 protocols in the IP layer or use only the default IPv4 protocols. The remainder of the TCP/IP stack is identical. Consequently, the same transport protocols, TCP UDP and SCTP, can run over both IPv4 and IPv6. Also, the same applications can run over both IPv4 and IPv6. Figure 11–4 shows how the IPv4 and IPv6 protocols work as a dual-stack throughout the various layers of the Internet protocol suite.
In the dual-stack scenario, subsets of both hosts and routers are upgraded to support IPv6, in addition to IPv4. The dual-stack approach ensures that the upgraded nodes can always interoperate with IPv4-only nodes by using IPv4.