ルーティング面でのIPv6移行（ルータからホストへの自動トンネリング）

   In some cases the source host may have direct connectivity to one or
   more IPv6-capable routers,  but the destination host might not have
   direct connectivity to any IPv6-capable router. In this case,
   provided that the destination host has an IPv4-compatible IPv6
   address, normal IPv6 forwarding may be used for part of the packet's
   path, and router to host tunneling may be used to get the packet from
   an encapsulating dual router to the destination host.

   In this case, the hard part is the IPv6 routing required to deliver
   the IPv6 packet from the source host to the encapsulating router. For
   this to happen, the encapsulating router has to advertise
   reachability for the appropriate IPv4-compatible IPv6 addresses into
   the IPv6 routing region.  With this approach, all IPv6 packets
   (including those with IPv4-compatible addresses) are routed using
   routes calculated  from native IPv6 routing. This implies that
   encapsulating routers need to advertise into IPv6 routing specific
   route entries corresponding to any IPv4-compatible IPv6 addresses
   that belong to dual hosts which can be reached in an neighboring
   IPv4-only region. This requires manual configuration of the
   encapsulating routers to control which routes are to be injected into
   IPv6 routing protocols.  Nodes in the IPv6 routing region would use
   such a route to forward IPv6 packets along the routed path toward the
   router that injected (leaked) the route, at which point packets are
   encapsulated and forwarded to the destination host using normal IPv4
   routing.

   Depending upon the extent of the IPv4-only and dual routing regions,
   the leaking of routes may be relatively simple or may be more
   complex.  For example, consider a dual Internet backbone, connected
   via one or two dual routers to an IPv4-only stub routing domain. In
   this case, it is likely that there is already one summary address
   prefix which is being advertised into the Internet backbone in order
   to summarize IPv4 reachability to the stub domain.  In such a case,
   the border routers would be configured to announce the IPv4 address
   prefix into the IPv4 routing within the backbone, and also announce
   the corresponding IPv4-compatible IPv6 address prefix into IPv6
   routing within the backbone.

   A more difficult case involves the border between a major Internet
   backbone which is IPv4-only, and a major Internet backbone which
   supports both IPv4 and IPv6. In this case, it requires that either
   (i) the entire IPv4 routing table be fed into IPv6 routing in the
   dual routing domain (implying a doubling of the size of the routing
   tables in the dual domain); or (ii) Manual configuration is required
   to determine which of the addresses contained in the Internet routing
   table include one or more IPv6-capable systems, and only these
   addresses be advertised into IPv6 routing in the dual domain.