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41935 Views 5 Replies Latest reply: Nov 2, 2010 1:07 AM by dmahler RSS

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Multicast - Convert IP to MAC

Jul 8, 2008 5:23 AM

colan haisell 13 posts since
Jun 26, 2008



Studying for BSCI an I am looking for a detailed example of how to convert a Multicast IP address to a Multicast MAC address. I have found several examples including my BSCI Student Guide and Cisco Press Exam Cert Guide but none of them show me a detailed bit by bit example of how this works.


Would appreciated a detailed bit by bit example.


Thanks in advance,



  • Currently Being Moderated
    2. Jul 8, 2008 6:56 AM (in response to colan haisell)
    Re: Multicast - Convert IP to MAC


    If you can obtain a copy of the CCIE R&S Exam Cert Guide v3, in Chapter 16, titled "Introduction to IP Multicasting", section 3 "Multicast IP Addresses" they provide a process with diagram on how to calculate the MAC from the IP.






    Not sure if Cisco is OK with me posting snippets from their books online, if they OK that then I can try to post it online for you.



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  • dmorrow 20 posts since
    Jul 27, 2008
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    4. Sep 5, 2008 9:25 PM (in response to colan haisell)
    Re: Multicast - Convert IP to MAC


    Hi Colan,



    Going on the MS article that Brent posted, which was of considerable help to me as well, I think a way to quickly calculate a corresponding overlap address would be as follows;



    If the 2nd Octet has a IP address value less than 128, then to calculate the corresponding overlap address you would simply add 128 (remember the rightmost bit in the 2nd Octet is ignored) only the last 23 bits are translated/considered.



    A random choice example;


 , overlap is, (42 + 128 = 172)



    and if I were given the address, I'd do a subtraction 172 - 128 =42



    therefore the same overlap would occur in any mulicast class D address e.g., rendering a 32:1 overlap



    224 to 239 = 16 Class D, therefore 16 (Class D's) x 2(overlaps in the 2nd octet) = 32 Overlaps



    I tested this on a number of addresses (in the 2nd octet) and got the same result as you would expected (no suprises).



    This calculation is reliant on the microsoft description Brent provided and their description made sense, so I hope my calculation is right.


















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  • dmahler 1 posts since
    Sep 23, 2008
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    5. Nov 2, 2010 1:07 AM (in response to dmorrow)
    Re: Multicast - Convert IP to MAC

    Replying to something 2 years later could be construed as silly but I wanted to comment on dmorrows post for the next person who wanders by and reads this chain like I did :-).


    So first - converting multicast IP to multicast MAC for dummies.

    1) Take the last 23 bits of the IP address in question (in binary)

    2) prepend it with a binary 0

    3) convert it to hex

    4) prepend 01:00:5e



    So an easier example: (OSPF hello)

    1) In binary this is 11100000.00000000.00000000.00000101 so the last 23 bits are 0000000.00000000.00000101

    2) Prepend a 0 to the last 23 - so this becomes : 00000000.00000000.00000101

    3) Convert this to hex which is 00:00:05

    4) Prepend 01:00:5e so this becomes 01:00:5e:00:00:05

    Oh so easy (sort of)


    The "32 overlaps" comes from this:

    When converting the IP you are only considering the last 23 bits, but IPs have 32 bits - so you are ignoring 9 bits.  However ALL multicast addresses start with 1110 in binary, so those don't count as being ignored.  So you are really ignoring 5 bits (9 minus 4 bits in 1110).  Ignoring 5 binary bits means there are 2^5 or 32 overlapping IP addresses that would all convert to the exact same multicast mac address.


    dmorrow is correct in that adding or subtracting 128 in the second octet can find you 1 example of any overlapping address - but there are 31 more examples :-)


    1110 [  0000.0  ]  0000000.00000000.00000101

    The bits in brackets above are being ignored so:


    These multicast IPs would all result in the same multicast MAC address:

    1110 [  0000.0  ]  0000000.00000000.00000101

    1110 [  0010.0  ]  0000000.00000000.00000101

    1110 [  1100.0  ]  0000000.00000000.00000101

    1110 [  0010.1  ]  0000000.00000000.00000101

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