2.2 Interpret Ethernet frame format

The IEEE 802.3 Ethernet Frame contains 7 fields. See below (source Wikipedia):

Screenshot from 2018-04-29 15-57-59

Here are the fields and their purpose:

  1. Preamble – 7 bytes – used for synchronizing the Layer 2 hardware (indicates that a Ethernet Frame follows)
  2. Start Frame Delimiter – 1 byte – Indicates that the next byte begins the Destination Mac Field
  3. Destination Mac Address  – 6 bytes – Identifies the destination Ethernet address
  4. Source Mac Address – 6 bytes – Identifies the source of the frame
  5. Length Or Type
    1. Length – 2 bytes – defines the length of the data field of the frame
    2. Type – 2 bytes – defines the type of protocol listed inside the frame
  6. Date and Pad – 46 to 1500 bytes – Hold the PDU/IP packet
  7. Frame Check Sequence (FCS) – 4 bytes – contains a CRC check of the frame – used to check the frame for errors.

 

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