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

    IPv6 Address Format with Interface ID and EUI-64

    John

      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

        • 1. Re: IPv6 Address Format with Interface ID and EUI-64
          Keith Barker - CCIE RS/Security, CISSP

          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. 

          • 2. Re: IPv6 Address Format with Interface ID and EUI-64
            Scott Morris - CCDE/4xCCIE/2xJNCIE

            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

            • 3. Re: IPv6 Address Format with Interface ID and EUI-64
              John

              Hi Keith

               

              The bit that confuses me is that on the address in the jpg on my first post they changed the start of the address to 02 ?  i assumed because the 7th bit flipped.

               

              Does that mean that if the 7th bit is a 1 = 0 when flipped the start of the address would be 00 ?

               

               

              Regards

              John

              • 4. Re: IPv6 Address Format with Interface ID and EUI-64
                John

                Hi

                 

                I have been looking at what you guys posted and i think i understand it now, tell me if i am right.

                 

                ======================================

                From the book

                For example, the following two lines list a host’s MAC address, and corresponding EUI-64

                format Interface ID, assuming the use of an address configuration option that uses the

                EUI-64 format:

                0034:5678:9ABC

                0234:56FF:FE78:9ABC

                =======================================

                So, the 0034 part is the start of the MAC address.

                Regardles of what it is, the 7th bit ends up either being a 1 or a 0

                In binary that is either 02 or 00 depending if it is a 1 or a 0

                So the start of every address will be either 00 or 02

                 

                Is that correct ?

                 

                 

                Regards

                 

                John

                • 5. Re: IPv6 Address Format with Interface ID and EUI-64
                  Keith Barker - CCIE RS/Security, CISSP

                  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

                  • 6. Re: IPv6 Address Format with Interface ID and EUI-64
                    Keith Barker - CCIE RS/Security, CISSP

                    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

                    • 7. Re: IPv6 Address Format with Interface ID and EUI-64
                      John

                      Keith/Scott

                       

                      Thank you, now i understand it.

                       

                       

                       

                       

                       

                      Regards

                       

                      John

                      • 8. Re: IPv6 Address Format with Interface ID and EUI-64
                        Keith Barker - CCIE RS/Security, CISSP

                        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.

                        • 9. Re: IPv6 Address Format with Interface ID and EUI-64
                          Sey

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