Figure 1 shows a router connected to three network segments. Each has a computer running IPv4 and IPv6 in dual-stack mode, which allows the two protocols to co-exist. Figure 2 shows two routers. Each router has a network segment on one of the Fast Ethernet ports.  There is a computer on each network segment that is running only IPv6.  There is an IPv4 only tunnel connecting the two routers. The tunnel encapsulates the IPv6 packet within an IPv4 packet for transmission. Figure 3 shows a router that has an IPv6 only segment on one Fast Ethernet port and an IPv4 only segment on another Fast Ethernet port.  The router is serving as a Network Address Translation 64 device allowing IPv6 enabled devices to communicate with IPv4 enabled devices.