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Destination address (DA)
— The DA field is used to identify which network device(s)
should receive the packet. It is a unique address. Please see the section of MAC
addressing.
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Source addresses (SA)
— The SA field indicates the source node. The SA is always an
individual address and the left-most bit in the SA field is always 0.
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Length/Type
— This field indicates either the number of the data bytes contained in the
data field of the frame, or the Ethernet type of data. If the value of first two bytes is less
than or equal to 1500 in decimal, the number of bytes in the data field is equal to the
Length/Type value, i.e. this field acts as Length indicator at this moment. When this field
acts as Length, the frame has optional fields for 802.3/802.2 SNAP encapsulation,
802.3/802.2 encapsulation and Netware 802.3 RAW encapsulation. Each of them has
different fields following the Length field.
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If the Length/Type value is greater than 1500, it means the Length/Type acts as Type.
Different type value means the frames with different protocols running over Ethernet being
sent or received.
For example,
0x0800 IP
datagram
0x0806 ARP
0x0835 RARP
0x8137 IPX
datagram
0x86DD IPv6
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Data
— Less than or equal to 1500 bytes and greater or equal to 46 bytes. If data is less
than 46 bytes, the MAC will automatically extend the padding bits and have the payload be
equal to 46 bytes. The length of data field must equal the value of the Length field when
the Length/Type acts as Length.
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Frame check sequence (FCS)
—
This field contains a 32-bit cyclic redundancy check
(CRC) value, and is a check sum computed with DA, SA, through the end of the data field
with the following polynomial.
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It is created by the sending MAC and recalculated by the receiving MAC to check if the
packet is damaged or not.