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  1. #1
    Join Date
    Mar 2008
    Posts
    1

    Lightbulb MAC Address I/G G/L bits

    Hello sir , it says in the book that the I/G , G/L bits are in the first octet, bits #46 and # 47 , i belive they are #40 and #41 , though in the wikipedia site it looked like b8 and b7 in the following pic

    http://en.wikipedia.org/wiki/Image:MAC-48_Address.svg


    from wikipedia
    by a network administrator, overriding the burned-in address. Locally administered addresses do not contain OUIs.
    Universally administered and locally administered addresses are distinguished by setting the second least significant bit of the most significant byte of the address. If the bit is 0, the address is universally administered. If it is 1, the address is locally administered. The bit is 0 in all OUIs. For example, 02-00-00-00-00-01. The most significant byte is 02h. The binary is 00000010 and the second least significant bit is 1. Therefore, it is a locally administered address.[2]
    any comments ?

  2. #2
    Join Date
    May 2008
    Location
    Asia
    Posts
    30

    Default

    well this may sound really odd but as a matter of fact both wikipedia and Todd are correct. Infact they are the two different representations that mac address has i.e canonical and non canonical

    Ok lemme describe em.

    canonical form:( this is what wikipedia describes)
    what u have see when u type ipconfig \all in the windows command prompt is the canonical form ( normal form as i call it ) in this case the
    global/local bits as well as the individual/ group bits are the last 2 bits of the 1st octet of mac address. example below

    If mac address is 12:34:56:78:9A: BC then

    mac:-------12-------- 34------ 56------- 78 ------- 9A---- BC
    canonical:00010 010 :00110100 :01010110 :01111000 :10011010:10111 100

    here the underlined bits are the global/local bits(1) and individual/group(0)

    non canonical form: (this is what Todd describes )

    ok now when u r transmitting this address on the network, we start by transmitting the least significant bits first hence on lan the address looks like this

    On LAN: 01001000: 00101100 :01101010: 00011110 : 01011001 :00111101

    tadaa !!!

    so u c how u have the individual/group bit (0) and the global/local bit (1) coming in first.

    so basically what happens is that the least significant bits become the most significant bits while transmitting.

    still confused refer RFC 2469

    ps: examples have been picked up from the rfc itself.



  3. #3
    Join Date
    Oct 2009
    Posts
    2

    Default Numbering scheme for MAC address bits.

    In either canonical or non-canonical form, the bits in a MAC address seem to be, in the CCNA study guide at least, numbered starting with 0, like below.

    47............. ............... ............... ............... ............... ............... ..........0
    00010010 :00110100 :01010110 :01111000 :10011010:10111 100


    This seems to be the only way to have the leftmost bits be numbers 46 and 47, rather than 47 and 48.

    Are the bits really numbered starting with 0?

  4. #4
    Join Date
    Oct 2009
    Posts
    2

    Default TCP and UDP headers numbered from bit 0 to bit 31

    Disregard my last post on this thread. Just saw on page 75 and 78 of the study guide that TCP and UDP headers are numbered from bit 0 to bit 31. I guess beginning with bit 0 is the convention for headers in general. Just didn't realize.

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