Tunneling mechanisms can be used to deploy an IPv6 forwarding infrastructure while the overall IPv4 infrastructure is still the basis. Tunneling can be used to carry IPv6 traffic by encapsulating it in IPv4 packets and tunneling it over the IPv4 routing infrastructure. For instance, if your provider still has an IPv4-only infrastructure, tunneling allows you to have a corporate IPv6 network and tunnel through your ISP’s IPv4 network to reach other IPv6 hosts or networks. The tunneling techniques and the encapsulation of IPv6 packets in IPv4 packets are defined in several RFCs, such as RFC 2473, 2893, and 3056, which differentiate two types of tunneling:
- Manually configured tunneling of IPv6 over IPv4
IPv6 packets are encapsulated in IPv4 packets to be carried over IPv4 routing infrastructures. These are point-to-point tunnels that need to be configured manually.
- Automatic tunneling of IPv6 over IPv4
IPv6 nodes can use different types of addresses, such as IPv4-compatible IPv6 addresses or 6to4 or ISATAP addresses, to dynamically tunnel IPv6 packets over an IPv4 routing infrastructure. These special IPv6 unicast addresses carry an IPv4 address in some of the IPv6 address fields.
How Tunneling Works
The concepts discussed in this paragraph apply to tunneling in general. The next two paragraphs discuss the difference between configured tunnels and automatic tunneling. Figure 10-1 shows two IPv6 networks connected through an IPv4-only network.
Figure 10-1. Encapsulation ...