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  1. Hi Rene

    Question, should the following sentence under Within Network say IPv4 rather than IPv6?

    “The destination IPv6 address network bits are derived from the BR IPv6 address that the router knows.”

    Also how does 100:10::1 get mapped to .1 on the IPv4 side?

    And on Outside Domain, why is the IPv4 Source the IP of CE2 (, when the ping is initiated from Host 1 behind CE1? Should it be


  2. Hi Chris,

    Thanks, that last line shouldn’t be there so I removed it.

    The IPv6 prefix has this format:

    Now take a look at this address:

    2001:DB8 is the ISP prefix, what comes next is the IPv4 address. in hexadecimal is C0A8:0101, but I don’t have to embed the entire IPv4 address. I can forget about 192.168.1. (those are the common bits), and only embed .1 in the IPv6 address, which is how we end up with 2001:DB8:100:

    What comes after 2001:DB8:100: is the subnet, I just picked one (:10).

    On the outside domain part, it should be (copy paste error, sorry :smile:) It has been fixed!


  3. Thanks Rene, any chance you could provide the configs at the end? I guess I’m wondering how the BR knows about the “General prefix” when you didn’t configure it.

    BR#show tunnel 6rd
    Interface Tunnel0:
    Tunnel Source:
    6RD: Operational, V6 Prefix: 2001:DB8::/32
    V4 Prefix, Length: 24, Value:
    V4 Suffix, Length: 0, Value:
    General Prefix: 2001:DB8:300::/40

  4. Hello Chris

    Let’s use this same topology for our example:
    This time however, we are dealing with the connection from the Internet to H1. Let’s imagine an Internet host H3 is communicating with H1.

    The ISP uses the subnet and each router has an IPv4 address. The hosts that are communicating have the following IP addresses:

    H1: 2001:DB8:100:10::1
    H3: 2001:AF98:773:14::1

    H3 sends an IPv6 packet destined for H1. Here’s what the encapsulated IPv6 packet looks like:


    When BR1 receives this packet, it checks the 6RD prefix of the destination address highlighted in red. It sees that it is indeed for this domain. It also looks at the third hextet to map to the correct IPv4 destination CE router so it knows where to send it. Notice also that the source IPv4 address is that of the BR1 router once the packet enters the domain.

    I hope this has been helpful!


  5. Hello ADP,

    2001:DB8:100::/40 is short for 2001:0db8:0100:0000:0000:0000:0000:0000/40. For this network address, the valid ranges are:

    up to

    2001:0DB8:01::/40 is short for 2001:0db8:0001:0000:0000:0000:0000:0000/40. (be aware of the leading zeroes) This is an address in the following range:

    up to

    See how this works? By specifying 2001:0DB8:01::/40, you get 2001:0DB8:0001::/40. An address of the 2001:0DB8:00::/40 network.

    Hope this helps :slight_smile: If not let me know.


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