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  1. Hello Brian

    MAC addresses when configured have the U/L bit which is the 7th bit of the 48 bit address. This bit, when set to 0 when this address is locally administered and 1 if the address is globally unique. An example is the virtual MAC address that is created by HSRP. This MAC address will always have 0 in the seventh bit, while a hardwired MAC address on a switch or a PC will have the 7th bit 1.

    Now because there is a mechanism of EUI-64 which is used to assign an IPv6 address that is derived from a MAC addresses, this L bit seems to have migrated into the IPv6 format. According to most sources, (and @ReneMolenaar can correct me if I’m wrong), this bit isn’t actually taken into account in most IPv6 applications. Both the FC00 and FD00 blocks are considered unique local addresses.

    The reason why the L should be changed to 1 is because the block FC00 has not been defined yet. It has been proposed to be managed by an allocation authority, but this has not gained acceptance in the IETF.

    I hope this has been helpful!


  2. Hi Rene,
    I have a silly question running over my head. I see that we have Global unicast address, somewhere i read that the range for global unicast is from 2000::/16 to 3fff::/16.
    My question:

    1. is this correct?
    2. why such a small range of global unicast addresses from a massive IPv6 address? Your introduction to IPv6 course mentioned that there is no real requirement of NAT in IPv6. With this small range we might require NAT in future (please correct me if i am wrong)
    3. can’t we assign other addresses like 7000::/16 as global unicast ?
      I know that IANA does the address allocation, but as i said it’s just a silly question troubling me in this course.
  3. Hi Nadav,

    The global IPv6 unicast address space must start with 001 (binary) so that’s correct yes:

    0010 (2000)
    0011 (3000)

    In other words, it has to start with 2000::/3. That covers everything in this range:


    That’s 42535295865117307932921825928971026432 addresses in total.

    For each person on the planet we get:

    42535295865117307932921825928971026432 / 7615097670 (world population) = 5.5856534e+27

    So, that’s a crazy number of IPv6 addresses only from the 2000::/3 range :smile:

    Here you can find the complete IPv6 allocation from IANA:

    And here’s the IANA global unicast address space:

    In RFC7249 we can read something about the allocation:

    The vast bulk of the IPv6 address space (approximately 7/8ths of the
    whole address space) is reserved by the IETF [RFC4291], with the
    expectation that further assignment of globally unique unicast
    address space will be made from this reserved space in accordance
    with future needs.

    0000::/8 and 0100::/8 are reserved for other purposes so that’s why we start with 2000:: for global addresses. It still feels like a waste to reserve an entire /8 that is probably never used but with this address space, we probably never run out.

    Here’s a funny website I ran into when I was looking some stuff up:


  4. Hello Nadav

    Anycast can indeed provide redundancy. With the growth of the Internet, many network services are using Anycast for high-availability requirements, such as DNS and content delivery networks. Anycast has grown in popularity for this purpose.

    Closest could mean multiple things. It could be on the basis of number of hops, physical distance, lowest cost based on a metric, latency, or based on the least congested route. Content delivery systems like Netflix may use it in order to have users obtain content from the closest possible (and thus fastest and least costly) server. This essentially brings the content closer to the end user.

    Only if there is a fault in configuration. IPv6 routers on the path of an Anycast packet through the network will not distinguish it from a unicast packet, but special handling is required from the routers near the destination (that is, within the scope of the Anycast address) as they are required to route an Anycast packet to the “nearest” interface within that scope. This is determined in much the same way that a routing protocol determines the path to a specific destination, based on whatever measure of distance (hops, cost, etc.) is being used.

    I hope this has been helpful!


  5. well received, thanks for these answers

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