Other Transition Mechanisms

The mechanisms that were specified previously handle interoperability between dual nodes and IPv4 nodes, if the dual nodes have an IPv4 address. The mechanisms do not handle interoperability between IPv6-only nodes and IPv4-only nodes. Also, the mechanisms do not handle interoperability between dual nodes that have no IPv4 address and IPv4-only nodes. Most implementations can be made dual. However, a dual implementation requires enough IPv4 address space to assign one address for every node that needs to interoperate with IPv4-only nodes.

Several possibilities enable you to accomplish this interoperability without requiring any new transition mechanisms.

• Use application layer gateways (ALG) that sit at the boundary between the IPv6-only nodes and the remainder of the Internet. Examples of ALGs in use today are HTTP proxies and mail relays.

• Companies are already selling network address translators (NAT) boxes for IPv4 that translate between the private IP addresses (for example, network 10—see RFC 1918) on the inside and other IP addresses on the outside. These companies will likely upgrade their NAT boxes to also support IPv6–to-IPv4 address translation.

Unfortunately, both ALG and NAT solutions create single points of failure. By using these solutions, the Internet becomes less effective. The IETF is working on a better solution for IPv6-only interoperability with IPv4-only nodes. One proposal is to use header translators with a way to allocate IPv4–compatible addresses on demand. Another proposal is to allocate IPv4–compatible addresses on demand and use IPv4 in IPv6 tunneling to bridge the IPv6-only routers.

The stateless header translator translates between IPv4 and IPv6 header formats if the IPv6 addresses in use can be represented as IPv4 addresses. The addresses must be IPv4-compatible or IPv4-mapped addresses. The support for these translators has been built into the IPv6 protocol. The translation can occur without any information loss, except for encrypted packets. Rarely used features such as source routing can produce information loss.