Transition Mechanisms
Translators

The general set of translating rules of datagram header is defined in RFC 2765: Stateless IP/ICMP Translation Algorithm (SIIT) and RFC 6145: IP/ICMP Translation Algorithm. Generally, these rules are used by all translators and they are very limited and do not support any extensions such as the IPv4 options or IPv6 extension headers. It means that translators drop these extensions.

Translators translate each datagram independently without any bindings to the previous datagrams and without preservation of data structures with information about the history or current status of ongoing communication. Passage of datagrams through SIIT (Stateless IP/ICMP Translation) translator is depicted in the figure below.

Passage of Datagrams Through SIIT Translator

Address mapping is part of the translating. That means the translation of IPv6 addresses to IPv4 addresses and vice versa. To do this the IPv4-embedded addresses are currently used. The 96-bit prefix is usually reserved for these addresses. It can be either an universal prefix 64:FF9B::/96 or prefix assigned by local ISP. IPv4 address follows the prefix. The IPv4-embedded address format is described in RFC 6052.

Translation of IPv4 address to IPv6 address can be made stateless. That means that IPv4 address is added for the IPv6 prefix. Otherwise, it is not that simple. There is most often used the dynamic mapping to translate IPv6 address to IPv4 address. This is similar to the NAT (Network Address Translation).