Where is the checksum "physically" in the Ethernet frame?
Every time I see an illustrative Ethernet frame image, it shows the checksum at the very end of the frame. It makes sense and I understand why he will be there. However, when I look at the packet in Wireshark, the CRC seems to be in front of the payload data in the frame. I am guessing that Wireshark is showing me the raw data that is on the wire. When transmitting a frame over Ethernet, does the CRC move in the bitstream somehow, or is Wireshark just not showing the exact bit placement? Or am I just not getting it right?
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FCS or Frame Check Sequence for ethernet, when transmitted over physical media, "physically" / "electrically" / "optically" is carried after the payload data in the frame. The FCS field is computed using a CRC skip on the source MAC address, target MAC address, length / type, data, and pad fields using 32-bit cyclic redundancy check (CRC). It is for these reasons that FCS occurs after the data field "physically ". Since most second layer devices want to be as small as possible, arranging the fields in this way makes it very easy for the receiving device to copy the entire frame to the buffer and then perform a CRC check before transmitting the frame to. CRC does not require reordering or parsing of the frame. Likely,most pdp ethernet diagrams show this to you correctly if FCS comes after payload / data. Wireshark just shows it in a different area, so user can evaluate all level 2 data before parsing level 3 data. This makes it easy to understand low level problem if crc doesn't match so that you parse level 2 information before proceeding.
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