ルーティング面でのIPv6移行（ホストからホストへの自動トンネリング）

   If both source and destination hosts make use of IPv4-compatible IPv6
   addresses, then it is possible for automatic tunneling to be used for
   the entire path from the source host to the destination host. In this
   case, the IPv6 packet is encapsulated in an IPv4 packet by the source
   host, and is forwarded by routers as an IPv4 packet all the way to
   the destination host. This allows initial deployment of IPv6-capable
   hosts to be done prior to the update of any routers.

   A source host may make use of Host to Host automatic tunneling
   provided that the following are both true:

     - the source address is an IPv4-compatible IPv6 address.
     - the destination address is an IPv4-compatible IPv6 address.
     - the source host does know of one or more neighboring IPv4-
       capable routers, or the source and destination are on the
       same subnet.

   If all of these requirements are true, then the source host may
   encapsulate the IPv6 packet in an IPv4 packet, using a source IPv4
   address which is extracted from the associated source IPv6 address,
   and using a destination IPv4 address which is extracted from the
   associated destination IPv6 address.

   Where host to host automatic tunneling is used, the packet is
   forwarded as a normal IPv4 packet for its entire path, and is
   decapsulated (i.e., the IPv4 header is removed) only by the
   destination host.