USB 3.0

Tunneling allows early IPv6 implementations to take advantage of existing IPv4 infrastructure without any change to IPv4 components. A dual-stack router or host on the “edge” of the IPv6 topology simply appends an IPv4 header to each IPv6 packet and sends it as native IPv4 traffic through existing links. IPv4 routers forward this traffic without knowledge that IPv6 is involved. On the other side of the tunnel, another dualstack router or host de-encapsulates the IPv6 packet and routes it to the ultimate destination using standard IPv6 protocols. To accommodate different administrative needs, IPv6 transition mechanisms include two types of tunneling: automatic and configured. To build configured tunnels, administrators manually define IPv6-to-IPv4

address mappings at tunnel endpoints. On either side of the tunnel, traffic is forwarded with full 128-bit addresses. At the tunnel entry point, a router table entry is defined manually to dictate which IPv4 address is used to traverse the tunnel. This requires a certain amount of manual administration at the tunnel endpoints, but traffic is routed through the IPv4 topology dynamically, without the knowledge of IPv4 routers. The 128-bit addresses do not have to align with 32-bit addresses in any way.