IPv6 Administration Guide

Tunneling Mechanism

To minimize any dependencies during the transition, all the routers in the path between two IPv6 nodes do not need to support IPv6. This mechanism is called tunneling. Basically, IPv6 packets are placed inside IPv4 packets, which are routed through the IPv4 routers. The following figure illustrates the tunneling mechanism through IPv4 routers (R).

Figure 4–2 Tunneling Mechanism

Illustrates how IPv6 packets that are placed inside IPv4 packets are tunneled through routers that use IPv4.

The different uses of tunneling in the transition follow:

A configured tunnel is currently used in the Internet for other purposes, for example, the MBONE, the IPv4 multicast backbone. Operationally, the tunnel consists of two routers that are configured to have a virtual point-to-point link between the two routers over the IPv4 network. This kind of tunnel is likely to be used on some parts of the Internet for the foreseeable future.

Automatic Tunnels

The automatic tunnels have a more limited use during early experimental deployment. Automatic tunnels require IPv4–compatible addresses. Automatic tunnels can be used to connect IPv6 nodes when IPv6 routers are not available. These tunnels can originate either on a dual host or on a dual router by configuring an automatic tunneling network interface. The tunnels always terminate on the dual host. These tunnels work by dynamically determining the destination IPv4 address, the endpoint of the tunnel, by extracting the address from the IPv4–compatible destination address.