3 Replies Latest reply: Feb 13, 2012 10:58 AM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

    IP multicast--Ethernet Multicast MAC Address Mapping



      I am started study of multicasing from Developing IP Multicast Networks, vol.1 book Page no. 51 and 52

      i am not able to understand below para. can you please explain me.?



      Ethernet Multicast MAC Address Mapping

      In the case of Ethernet, IP multicast frames all use MAC layer addresses beginning with the

      24-bit prefix of 0x0100.5Exx.xxxx. Unfortunately, only half of these MAC addresses are

      available for use by IP multicast. This leaves 23 bits of MAC address space for mapping

      Layer 3 IP multicast addresses into Layer 2 MAC addresses. Since all Layer 3 IP multicast

      addresses have the first 4 of the 32 bits set to 0x1110, this leaves 28 bits of meaningful IP

      multicast address information. These 28 bits must map into only 23 bits of the available

      MAC address. This mapping is shown graphically in Figure 2-3.


      Just 23 Bits?

      There’s an interesting story as to why only 23 bits worth of MAC address space was

      allocated for IP multicast. Back in the early 1990s, Steve Deering was bringing some of his

      research work on IP multicasting to fruition, and he wanted the IEEE to assign 16

      consecutive Organizational Unique Identifiers (OUIs) for use as IP multicast MAC

      addresses. Because one OUI contains 24 bits worth of address space, 16 consecutive OUI’s

      would supply a full 28 bits worth of MAC address space and would permit a one-to-one

      mapping of Layer 3 IP multicast addresses to MAC addresses. Unfortunately, the going

      price for an OUI at the time was $1000 and Steve’s manager, the late Jon Postel, was unable

      to justify the $16,000 necessary to purchase the full 28 bits worth of MAC addresses.

      Instead, Jon was willing to spend $1000 to purchase one OUI out of his budget and give

      half of the addresses (23 bits worth) to Steve for use in his IP multicast research.



      thanks and regards,


        • 1. Re: IP multicast--Ethernet Multicast MAC Address Mapping
          Keith Barker - CCIE RS/Security, CISSP

          Hello Rocky-


          Steve's boss didn't have the budget. 


          Here is how we can sort all the numbers out:


          Multicast (class D) begins with 1110 for the IP.    That is 4 bits out of 32, leaving 28 bits left.

          This is old news.


          According to the story above, Postel only had the budget for 1 OUI  which is 48 bits and a person who purchases one is assigned the the first 24 bits, and the vendor can do whatever they want with the last 24 bits.  Postel purchased this one (first 6 characters = the first 24 bits):


          And he could do anything he wants with the last 24 bits.

          The range would have been:

          01-00-5E-00-00-00 through 01-00-5E-FF-FF-FF


          Because an IPv4 class D (multicast address) begins (in binary) as 1110, there are 28 bits left in the IPv4 address.   If Jon had purchased 16 consecutive OUIs, for example:



          all the way through


          (for a total of 16 OUIs)


          Then the last 28 bits of the OUI would all be available for the use by the vendor (in this case for use by Multicast).    With the first 4 bits of an IPv4 address locked in at 1110, the last 28 bits of the multicast IPv4 group address address could exactly fit into the last 28 bits of the OUI ranges purchased (see above) to be used as the Layer 2 address, and there would be a one to one mapping.




          Now for Steve Deering.   Jon only purchased/registered 1 OUI.  So the one to one mapping was out of the question.    To add insult to injury,  Jon gave 1/2 of the one OUI purchased for Steve to use for his "multicast project".   Jon told Steve he could use the range of:


          01-00-5E-00-00-00  through   01-00-5E-7F-FF-FF

          color coded binary:

          01-00-5E-0000 0000-00-00  through   01-00-5E-0111 1111-FF-FF



          Which is where we get the 23 bits worth that Steve had to make everything fit in.  The single leading zero bit in red (above) is not available for Steve to change.


          The half that Steve didn't get to use was

          01-00-5E-80-00-00  through   01-00-5E-FF-FF-FF


          In short, this results in several IPv4 multicast groups that end up mapping to the same L2 address, so planning is required within an organization to select IPv4 groups that are far enough away from each other as to not end up with the same L2 address used.


          Here is a video on the conversion.


          Best wishes,





          • 2. Re: IP multicast--Ethernet Multicast MAC Address Mapping

            Back to the origin, amazing, thx for sharing the story !


            If i understand well (i've bad english), the moral of the story is J. tightwad in 90's (i'm kidding of course)


            Perhaps a rumor but i heard that the (IPv6 to Ethernet) multicast address was allocated by J. when i goes to the doctor and the doctor ask him to say " 33 33 "





            • 3. Re: IP multicast--Ethernet Multicast MAC Address Mapping
              Scott Morris - CCDE/4xCCIE/2xJNCIE

              hehehe...  yes, there's much more overlap going from IPv6 to MAC and the "33 33" beginning, leaving 24 bits.


              IPv6 has 112 bits possible for the group address portion, so there's a LOT of possibility for overlaps no matter how you slice things.