ルーティング面でのIPv6移行（例）

   Figure 2 illustrates an example network with two regions A and B.
   Region A is dual, meaning that the routers within region A are
   capable of forwarding both IPv4 and IPv6. Region B is IPv4-only,
   implying that the routers within region B are capable of routing only
   IPv4. The illustrated routers R1 through R4 are dual. The illustrated
   routers r5 through r9 are IPv4-only. Also assume that hosts H3
   through H8 are dual. Thus H7 and H8 have been upgraded to be IPv6-
   capable, even though they exist in a region in which the routers are
   not IPv6-capable. However, host h1 and h2 are IPv4-only.

     .........................       .......................
     .                       .       .                     .
     .       h1              .       .              |-h2   .
     .       |               .       .              |      .
     .  H3---R1--------R2---------------r5----r9----+      .
     .       |         |     .       .        |     |-H7   .
     .       |         |     .       .        |            .
     .       |         |     .       .        |            .
     .  H4---R3--------R4---------------r6----r8-----H8    .
     .                       .       .                     .
     .........................       .......................
      Region A (Dual Routers)        Region B (IPv4-only Rtrs)

                Figure 2: Example of Automatic Tunneling

   Consider a packet from h1 to H8. In this case, since h1 is IPv4-only,
   it will send an IPv4 packet. This packet will traverse regions A and
   B as a normal IPv4 packet for the entire path. Routing will take
   place using normal IPv4 routing methods, with no change from the
   operation of the current IPv4 Internet (modulo normal advances in the
   operation of IPv4, of course). Similarly, consider a return packet
   from H8 to h1. Here again H8 will transmit an IPv4 packet, which will
   be forwarded as a normal IPv4 packet for the entire path.

   Consider a packet from H3 to H8. In this case, since H8 is in an
   IPv4-only routing domain, we can assume that H8 uses an IPv4-
   compatible IPv6 address. Since both source and destination are IPv6-
   capable, H3 may transmit an IPv6 packet destined to H8. The packet
   will be forwarded as far as R2 (or R4) as an IPv6 packet.

   Router R2 (or R4) will then encapsulate the full IPv6 packet in an
   IPv4 header for delivery to H8. In this case it is necessary for
   routing of IPv6 within region A to be capable of delivering this
   packet correctly to R2 (or R4). As explained in section 3.3, routers
   R2 and R4 may inject routes to IPv4-compatible IPv6 addresses into
   the IPv6 routing used within region A corresponding to the routes
   which are available via IPv4 routing within region B.

   Consider a return packet from H8 to H3. Again, since both source and
   destination are IPv6-capable, a IPv6 packet may be transmitted by H8.
   However, since H8 does not have any direct connectivity to an IPv6-
   capable router, H8 must make use of an automatic tunnel.  Which form
   of automatic tunnel will be used depends upon the type of address
   assigned to H3.

   If H3 is assigned an IPv4-compatible address, then the requirements
   specified in section 3.3.1 will all be satisfied. In this case host
   H8 may encapsulate the full IPv6 packet in an IPv4 header using a
   source IPv4 address extracted from the IPv6 address of H8, and using
   a destination IPv4 address extracted from the IPv6 address of H3.

   If H3 has an IPv6-only address, then it is not possible for H8 to
   extract an IPv4 address to use as the destination tunnel address from
   the IPv6 address of H3.  In this case H8 must use host to router
   tunneling, as specified in section 3.3.2. In this case one or both of
   R2 and R4 must have been configured with a tunnel endpoint IPv4
   address (R2 and R4 may use either the same address or different
   addresses for this purpose).  R2 and/or R4 therefore advertise
   reachability to the tunnel endpoint address to r5 and r6
   (respectively), which advertise this reachability information into
   region B. Also, H8 must have been configured to know which tunnel
   endpoint address to use for host to router tunneling. This will
   result in the IPv6 packet, encapsulated in an IPv4 header, to be
   transmitted as far as the border router R2 or R4. The border router
   will then strip off the IPv4 header, and forward the remaining IPv6
   packet as a normal IPv6 packet using the normal IPv6 routing used in
   region A.