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Ethernet frame

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Needix59 Posted messages 469 Status Member -  
Needix59 Posted messages 469 Status Member -
Hello,

I really have a hard time grasping certain concepts about the Ethernet frame, here’s what I know:



I know this by heart, but then when I come across an image that tells me this:



I’m a bit confused... why is there no preamble in this frame !! :(

And worse:



Here I’m completely lost, why are the @IP dst and @IP src found in this damned frame? This is not at all what I learned :/

Can you explain this to me?

Thanks in advance!

Best regards
N.

1 answer

  1.
    brupala Posted messages 111140 Registration date   Status Member Last intervention   14 441
     
    Hi,
    you will have to keep learning ...
    The Ethernet preamble exists and is taught in courses, but you will never see it in a frame capture; it is fortunately removed by the network card, just like the CRC/FCS.
    Anyway, if the FCS is incorrect, the frame is discarded and no one talks about it anymore.
    The last diagram is wrong or too simplified, but it's true that you can see the IP addresses if there are any in a frame, by analyzing a bit further, then the TCP/UDP ports and other upper-level protocol data, if there are any; all that is in the data field of Ethernet/802.3.
    You should install Wireshark and get an idea for yourself about decoding the different levels.

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    1. Needix59 Posted messages 469 Status Member 66
       
      Which physical device creates the segments (PDU layer 4)
      Which physical device creates the packets (PDU layer 3)
      Which physical device creates the frames (PDU layer 2)

      And if I understood correctly from the courses I've seen online: segment < packet < frame (the frame contains the packet which in turn contains the segment) is that right? Or am I completely wrong?

      Thank you for taking the time to respond!
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    2. brupala Posted messages 111140 Registration date   Status Member Last intervention   14 441 > Needix59 Posted messages 469 Status Member
       
      The layers 3/4 are managed by the OS.
      Layer 2 is managed by the network card for most functions, especially encapsulation, segmentation, and error control, and by the OS for some (ARP for example).
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