ルーティング面でのIPv6移行（ホストからルータへのデフォルトトンネリング設定）

   In some cases "configured default" tunneling may be used to
   encapsulate the IPv6 packet for transmission from the source host to
   an IPv6-backbone. However, this requires that the source host be
   configured with an IPv4 address to use for tunneling to the backbone.

   Configured default tunneling is particularly useful if the source
   host does not know of any local IPv6-capable router (implying that
   the packet cannot be forwarded as a normal IPv6 packet directly over
   the link layer), and when the destination host does not have an
   IPv4-compatible IPv6 address (implying that host to host tunneling
   cannot be used).

   Host to router configured default tunneling may optionally also be
   used even when the host does know of a local IPv6 router. In this
   case it is a policy decision whether the host prefers to send a
   native IPv6 packet to the IPv6-capable router or prefers to send an
   encapsulated packet to the configured tunnel endpoint.

   Similarly host to router default configured tunneling may be used
   even when the destination address is an IPv4-compatible IPv6 address.
   In this case for example a policy decision may be made to prefer
   tunneling for part of the path and native IPv6 for part of the path,
   or alternatively to use tunneling for the entire path from source
   host to destination host.

   A source host may make use of host to router configured default
   tunneling provided that ALL of the following are true:

     - the source address is an IPv4-compatible IPv6 address.
     - the source host does know of one or more neighboring IPv4-
       capable routers
     - the source host has been configured with an IPv4 address of
       an dual router which can serve as the tunnel endpoint.

   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 corresponds to the
   configured address of the dual router which is serving as the tunnel
   endpoint.

   When host to router configured default tunneling is used, the packet
   is forwarded as a normal IPv4 packet from the source host to the dual
   router serving as tunnel endpoint, is decapsulated by the dual
   router, and is then forwarded as a normal IPv6 packet by the tunnel
   endpoint.

   The dual router which is serving as the end point of the host to
   router configured default tunnel must advertise reachability into
   IPv4 routing sufficient to cause the encapsulated packet to be
   forwarded to it.

   The simplest approach is for a single IPv4 address to be assigned for
   use as a tunnel endpoint.  One or more dual routers,  which have
   connectivity to the IPv6 backbone and which are capable of serving as
   tunnel endpoint,  advertise a host route to this address into IPv4
   routing in the IPv4-only region.  Each dual host in the associated
   IPv4-only region is configured with the address of this tunnel
   endpoint and selects a route to this address for forwarding
   encapsulated packet to a tunnel end point  (for example, the nearest
   tunnel end point, based on whatever metric(s) the local routing
   protocol is using).

   Finally, in some cases there may be some reason for specific hosts to
   prefer one of several tunnel endpoints, while allowing all potential
   tunnel endpoints to serve as backups in case the preferred endpoint
   is not reachable. In this case, each dual router with IPv6 backbone
   connectivity which is serving as potential tunnel endpoint is given a
   unique IPv4 address taken from a single IPv4 address block (where the
   IPv4 address block is assigned either to the organization
   administering the IPv4-only region, or to the organization
   administering the local part of the IPv6 backbone). In the likely
   case that there are much less than 250 such dual routers serving as
   tunnel endpoints, we suggest using multiple IPv4 addresses selected
   from a single 24-bit IPv4 address prefix for this purpose. Each dual
   router then advertises two routes into the IPv4 region: A host route
   corresponding to the tunnel endpoint address specifically assigned to
   it, and also a standard (prefix) route to the associated IPv4 address
   block. Each dual host in the IPv4-only region is configured with a
   tunnel endpoint address which corresponds to the preferred tunnel
   endpoint for it to use. If the associated dual router is operating,
   then the packet will be delivered to it based upon the host route
   that it is advertising into the IPv4-only region. However, if the
   associated dual router is down, but some other dual router serving as
   a potential tunnel endpoint is operating, then the packet will be
   delivered to the nearest operating tunnel endpoint.