Multistage Interconnection Networks (MINs) provide efficient communication resources between network components in an appealing cost/performance relation. Consequently, they have been widely used on Internet connections such as ATM and gigabit Ethernet switches, terabit routers as well as for interconnecting parallel systems. In this paper a novel analytical method for providing performance evaluation on MINs is presented under multicast environment. The proposed analysis was applied on various network size MINs constructed by variable buffer-length Switching Elements (SEs) per network stage, supporting mixed traffic patterns such as unicast and multicast, under various offered loads in uniform traffic conditions. The applied multicasting policy, where a packet is copied and transmitted when only both destination buffers are available has not been considered insofar by former models. This mechanism, which is called "full-multicast" was employed by all SEs using the "Cell Replication While Routing" (CRWR) technique.The proposed architecture, which is thoroughly analyzed by our novel analytical model, can be also useful in performance study and development of other internet communication links supporting multicast traffic.