ルーティング面でのIPv6移行（自動トンネル）

   Automatic tunneling may be used when both the sending and destination
   nodes are connected by IPv4 routing.  In order for automatic
   tunneling to work, both nodes must be assigned IPv4-compatible IPv6
   addresses.  Automatic tunneling can be especially useful where either
   source or destination hosts (or both) do not have any adjacent IPv6-
   capable router.  Note that by "adjacent router", this includes
   routers which are logically adjacent by virtue of a manually
   configured point-to-point tunnel (which is treated as if it is a
   simple point-to-point link).

   With automatic tunneling, the resulting IPv4 packet is forwarded by
   IPv4 routers as a normal IPv4 packet, using IPv4 routes learned from
   routing protocols. There are therefore no special issues related to
   IPv4 routing in this case. There are however routing issues relating
   to how IPv6 routing works in a manner which is compatible with
   automatic tunneling, and how tunnel endpoint addresses are selected
   during the encapsulation process.  Automatic tunneling is useful from
   a source host to the destination host, from a source host to a
   router, and from a router to the destination host. Mechanisms for
   automatic tunneling from a router to another router are not currently
   defined.