TRANSMISSION LAYER for 802.x protocols


                       1       2       3       3       4
        0      7       5       3       1       9       7
       +------------------------------------------------+
       |          Destination MAC Address               |
       +------------------------------------------------+
       |            Source MAC Address                  |
       +---------------+--------------------------------+
       | Protocol Type | 
       +-------+-------+
ARP 
PACKET                 1       2       3       3       4
DATA    0      7       5       3       1       9       7
       +---------------+---------------+----------------+
       |      HRD      |      PRO      |  HLN  |   PLN  |
       +---------------+---------------+----------------+
       |      OP       |      SHA ...
       +------------------------------------------------+
       |      SPA ...
       +------------------------------------------------+
       |      THA ...
       +------------------------------------------------+
       |      TPA ...
       +------------------------------------------------+
       |      SHA ...

Ethernet transmission layer 
(not necessarily accessible to the user):
  DESTINATION     48.bit: Destination Address
  SOURCE          48.bit: Source Address
  PROTOCOL TYPE   16.bit: Protocol type
                          (set to ARP).

Ethernet packet data:
HRD   16.bit: Hardware address space 
              (e.g., Ethernet, Packet Radio Net.)
PRO   16.bit: Protocol address space.  
              For Ethernet hardware, this is from
              the set of type fields ether_typ$.
HLN    8.bit: Hardware Address Length (0-255 Bytes)
PLN    8.bit: Protocol Address Length (0-255 Bytes)
OP    16.bit: Opcode: either request or reply
SHA   nbytes: Sender Hardware Address (this packet), 
              n from the HLN field.
SPA   mbytes: Sender Protocol Address (this packet), 
              m from the PLN field.
THA   nbytes: Target Hardware Address (this packet),
              (if known).
TPA   mbytes: Target Protocol Address