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Two sites that run only IPv6 are to be connected through a network that runs only IPv4, and the traffic between the sites should remain IPv6 packets. Which method is appropriate?

  1. AGive the hosts at the sites IPv4 addresses as well, making them dual stack, and have them communicate between the sites over IPv4
  2. BIncrease the MTU on the routers in the IPv4 network so that they forward the IPv6 packets as they are
  3. CBuild a tunnel in which the routers at each site's exit put IPv6 packets inside IPv4 packets for the journey and take them out at the other site
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Answer: C. Build a tunnel in which the routers at each site's exit put IPv6 packets inside IPv4 packets for the journey and take them out at the other site

A tunnel carries packets of one protocol inside the data part of packets of another protocol. In an IPv6-over-IPv4 tunnel, the routers in the IPv4 network see ordinary IPv4 packets, so the traffic can be carried even though the equipment along the way does not support IPv6. Dual stack runs both IPv4 and IPv6 on one device and is widely used while IPv4 and IPv6 coexist, but in this case the traffic between the sites would be IPv4. A tunnel adds headers, so the packets that can be carried inside it are smaller by that amount.

  • A. The traffic between the sites would be IPv4, which does not meet the requirement that it stay IPv6.
  • B. Routers that run only IPv4 cannot interpret and forward IPv6 packets, whatever the MTU.
  • C. Correct. IPv6 packets are carried inside IPv4 packets.
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