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3098 Views 9 Replies Latest reply: Aug 20, 2011 9:27 PM by Sey RSS

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IPv6 Address Format with Interface ID and EUI-64

May 10, 2011 12:13 PM

John 2,289 posts since
Jan 17, 2009

Hi

 

I have been off my studies for 3 months due to work commitments, amazing how much you can forget in that time, I am back on my studies now every day and I will try very hard not to miss any.

 

This example here is confusing me as I have seen other examples which are different. If we change the 7th digit to 1 (00000010) then it will always be binary 02 so do we change ALL 1st 2 digits to 02 on all addresses ?

 

 

 

 

 

Regards

 

 

 

John

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  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    1. May 10, 2011 8:55 PM (in response to John)
    Re: IPv6 Address Format with Interface ID and EUI-64

    Hello John-

     

    On Cisco routers, for the EUI address, it inverts the 7th bit from what it was, regardless if it was a 1 or 0.

     

    If it was a 0, it flips it to a one.   If it was a 1, it flips it to a 0.

     


    NDC-R1(config)#int fa 0/0

    NDC-R1(config-if)#mac-address 0011.1111.1111

    NDC-R1(config-if)#ipv6 enable

    NDC-R1(config-if)#do show ipv6 int brief

    FastEthernet0/0            [up/up]

        FE80::0211:11FF:FE11:1111


    NDC-R1(config-if)#no ipv6 enable           

    NDC-R1(config-if)#mac-address 0211.1111.1111

    NDC-R1(config-if)#ipv6 enable              

    NDC-R1(config-if)#do show ipv6 int brief   

    FastEthernet0/0            [up/up]

        FE80::0011:11FF:FE11:1111


     

    So the bottom line is that the bit, on or off, doesn't mean anything anymore, but at some historical point it probably did.

     

    Best wishes,

     

    Keith

     

    Message was edited by: Keith to bold the correct hex characters. 

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,398 posts since
    Oct 7, 2008
    Currently Being Moderated
    2. May 10, 2011 8:22 PM (in response to John)
    Re: IPv6 Address Format with Interface ID and EUI-64

    It's an academic exercise (brought about by people who don't think in reality IMHO!).

     

    Anyway, the least significant bit of the most significant byte (rightmost bit of leftmost byte) on a MAC address is known as the I/G bit for Individual (0) or Group (1) address.

     

    The second bit there is known as the U/L bit for a Universally Assigned (0) or Locally Assigned (1) address.  In the old days of token ring, this was called LAA or Locally Assigned Address used for grouping/role-based functionality.

     

    With EUI-64, if you can't figure out that the address has changed when it grows from 48 bits to 64 bits, you have some serious perception issues!    And yet we flip it anyway.

     

    Following the original theory, it should ALWAYS be a "1" value to indicate change.  However, when writing the IPv6 specs, someone just says "should change" and coders interpreted that to mean if 0, then 1...  If 1, then 0...

     

    Go figure.

     

    HTH,

     

    Scott

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    5. May 11, 2011 1:54 PM (in response to John)
    Re: IPv6 Address Format with Interface ID and EUI-64

    Hello John-

     

    Perhaps a few more examples would shed more light on the subject.

     

    Here we go:

     

    0000 0000   which is 0 0 in hex

    would become

    0000 0010   which is 0 2 in hex


    Another example:


    0000 0101   which is 0 5 in hex

    would become

    0000 0111   which is 0 7 in hex

     

    One more:

     

    0000 1110   which is 0 E in hex

    would become

    0000 1100   which is 0 C in hex


    Best wishes,


    Keith

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    6. May 11, 2011 2:08 PM (in response to John)
    Re: IPv6 Address Format with Interface ID and EUI-64

    NDC-R5(config)#int fa 0/0

    NDC-R5(config-if)#mac-address 0034.5678.9ABC

    NDC-R5(config-if)#ipv6 enable

    NDC-R5(config-if)#do show ipv6 int brief

     

        FE80::234:56FF:FE78:9ABC

    ! Note: leading 0 is dropped before the 2 in the address.


    Original MAC address:

    0000 0000  = in hex 00


    flipping bit 7, to change it:

    0000 0010  = in hex 02

     

    The reality is, that on Cisco IOS, it doesn't allow for a configured mac address to have the least significant bit of the first byte, being odd.   So beginning there, the only valid hex characters to begin with for bits 5-8 (from the MAC address) would be 0,2,4,6,8,A,C,E

     

    As a result, the 2nd nibble, (bits 5-8) would always be even, and would end up being 0,2,4,6,8,A,C,E (even).

     

    In an earlier post, a few minutes ago, I showed some possible outcomes.

     

    Keith

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    8. Aug 20, 2011 2:15 PM (in response to John)
    Re: IPv6 Address Format with Interface ID and EUI-64

    Hi John,

     

    In a recent video, I included a piece on the EUI and that 7th inverted bit on a Cisco router.   If interested, it is at the 10 minute mark in the video.

     

    Best wishes,

     

    Keith

     

     

    Best viewed full screen, using YouTube's HD 720p option.

  • Sey 1,388 posts since
    May 4, 2010

    Great video and great discussion. Now I know why we flip that bit. Thank you all.

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