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  1. This is killing me. I have a task to convert 239.192.0.1 to a mac-address. I am just not getting it right now.

    01:00:5e:ef: is as far as I get…how to I convert the 100 (binary for 192 is 11000000 so I eliminate the first 1) to hex? 01:00:5e:ef:?0:00:01 is where I am at.

  2. Hi James,

    I understand that these calculations make your head spin…they are kinda annoying :slight_smile:

    Here’s how to figure out what MAC address your IP address maps to:

    1. We convert your IP address to binary:

    decimal: 239.192.0.1
    binary: 11101111 11000000 00000000 00000001

    1. We only care about the last 23 bits of your IP address so I removed the first 9 bits, that gives us:

    1000000 00000000 00000001

    1. Now we need to convert these 23 bits into hexadecimal. You need to take 4 binary bits and convert them to hexadecimal:

    binary: 100 0000 0000 0000 0000 0001
    hex: 4 0 0 0 0 1

    That gives us:

    40:00:01

    The first part of a multicast MAC address always starts with 01:00:5e so let’s put that in front:

    01:00:5e + 40:00:01 = 01:00:5e:40:00:01 (your multicast MAC address).

    Does that help?

    Rene

  3. Hi Rene,

    Excellent post and excellent explanation of what should be simpler and was made unnecessarily complex :slight_smile:

    Just one small correction:

    Where you say “The multicast IP addresses above all map to the same multicast MAC address (01-00-5E-01-01)”, I think you’re missing one octet. I believe it should be 01-00-5E-01-01-01.

  4. Hi Rene,

    Excellent creation as usual . Please confirm my understanding is correct which is grabbed from this lesson. Please let me know if I am wrong :slight_smile:

    General format of MAC address is MM:MM:MM:SS:SS:SS.The leftmost 6 digits (24 bits : MM:MM:MM ) called a “prefix” is associated with the adapter manufacturer.
    The rightmost digits (24 bits : SS:SS:SS) of a MAC address represent an identification number for the specific device with the same vendor prefix.

    But here in this multicast we use the leftmost 3 octets are (24 bits : MM:MM:MM) always 01-00-5E and the right most digits(24 bits : SS:SS:SS) will be a single OUI (24bits). From this single OUI(24 bits) we use only half of this 24-bits which means 23 bits can be used for mapping 28 bits Mutlicast IP addresses.

  5. Hello Chris

    You are correct that these layer two addresses are indeed multicast addresses. However, these function for exclusively layer two protocols such as PVST, STP, CDP, VTP, UDLD and others. This means that there is no corresponding multicast IP address to map them to. Multicast IP addresses are only mapped using the stated rule.

    I hope this has been helpful!

    Laz

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