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Forum Replies

  1. Hi Jose,

    Let’s look at an example:

    R1#show interfaces GigabitEthernet 1 | include bia
      Hardware is CSR vNIC, address is fa16.3e60.0217 (bia fa16.3e60.0217)

    The 7th bit is in the first two hexadecimal characters:

    fa = 1111 1010

    As you can see, the 7th bit is set to 1 here. Now let’s check the IPv6 address:

    R1#show ipv6 interface GigabitEthernet 1 | include link-local
      IPv6 is enabled, link-local address is FE80::F816:3EFF:FE60:217

    Let’s write down the complete uncompressed address:


    Let’s look only at the EUI-64 part:


    We only care about the first two hexadecimal characters:

    F8 = 1111 1000

    As you can see, the 7th bit has been inverted from 1 to 0.

    Let’s see what happens when we change the 7th bit in the MAC address, right now it starts with fa:

    fa = 1111 1010

    Let’s make it:

    f8 = 1111 1000

    R1(config)#interface gigabitEthernet 1
    R1(config-if)#mac-address f816.3e60.0217

    Our IPv6 address has become:

    R1#show ipv6 interface GigabitEthernet 1 | include link-local
      IPv6 is enabled, link-local address is FE80::FA16:3EFF:FE60:217

    The complete uncompressed address is:


    Here’s the EUI-64 part:


    First two hexadecimal characters:

    FA = 1111 1000

    As you can see, it got inverted. No matter if the 7th bit of the MAC address is a 0 or 1, it always gets inverted.

    Hope this helps!


  2. Hi Diana,

    Here’s an example:

    R1#show ipv6 interface GigabitEthernet 0/1
    GigabitEthernet0/1 is up, line protocol is up
      IPv6 is enabled, link-local address is FE80::F816:3EFF:FEE6:7777

    That’s the IPv6 link-local address that uses EUI-64. You can configure it manually if you want:

    R1(config)#int gi0/1                                                      
    R1(config-if)#ipv6 address FE80:1111:1111:1111:ABCD:ABCD:ABCD:ABCD link-local
    R1#show ipv6 interface GigabitEthernet 0/1 | include FE80
      IPv6 is enabled, link-local address is FE80:1111:1111:1111:ABCD:ABCD:ABCD:ABCD

    Here’s a global unicast address with EUI-64:

    R1(config-if)#ipv6 address 2001:1111:1111:1111::/64 eui-64

    R1#show ipv6 interface GigabitEthernet 0/1 | include 2001:
        2001:1111:1111:1111:F816:3EFF:FEE6:7777, subnet is 2001:1111:1111:1111::/64

    If you don’t want to use EUI-64 then you can configure global unicast addresses yourself:

    R1(config-if)#ipv6 address 2001:1111:1111:1111:ABCD:ABCD:ABCD:ABCD/64

    R1#show ipv6 interface GigabitEthernet 0/1 | include ABCD 
        2001:1111:1111:1111:ABCD:ABCD:ABCD:ABCD, subnet is 2001:1111:1111:1111::/64 

    That’s it. Hope this helps.


  3. Hi Joel,

    First of all, I wouldn’t worry too much about this too much. The bit flipping is just one minor sub-topic of all IPv6 related stuff you can expect in the exam. It’s possible that you don’t get any questions about it so don’t stress about it too much. You can expect a lot of regular subnetting questions so that’s why I highly recommend a “cheat sheet” for that.

    It seems this list would work. It might be a quick method if you’d get 10 bit flipping questions but if you only would get 1 question, it might be just as fast to quickly calculate it?

    hex > binary > flip 7th bit> hex

    So for example:

    hex: 12 =binary: 0001 0010

    flip 7th bit:

    binary: 0001 0000

    back to hex:


    And we are done.

  4. Hi Rene,
    In the case of multiple IPv6 addresses on a single interface, is it still possible to use the EUI-64 and how?

  5. Hello Nadav,

    That is no problem, the prefix is different so you will have a unique 128-bit IPv6 address. Here’s an example:

    R1(config)#interface GigabitEthernet 0/1
    R1(config-if)#ipv6 address 2001:DB8:1:1::/64 eui-64 
    R1(config-if)#ipv6 address 2001:DB8:2:2::/64 eui-64 
    R1(config-if)#ipv6 address 2001:DB8:3:3::/64 eui-64

    Gets you:

    R1#show ipv6 interface brief | include 2001


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