Describe ARP message format in brief.
0 8 16 24 31
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H A R D W A R E A D D R E S S T Y P E
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P R O T O C O L A D D R E S S T Y P E
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H A D D E R L E N
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P A D D E R L E N
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O P E R A T I O N
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S E N D E R H A D D E R ( f i r s t 4 o c t e t s )
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S E N D E R H A D D E R ( l a s t 2 o c t e t s )
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S E N D E R P A D D E R ( f i r s t 2 o c t e t s )
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S E N D E R P A D D E R ( l a s t 2 o c t e t s )
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T A R G E T H A D D E R ( f i r s t 2 o c t e t s )
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T A R G E T H A D D E R ( l a s t 4 o c t e t s )
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T A R G E T p A D D E R ( a l l 4 o c t e t s )
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The Format of an ARP Message
The first two 16-bit fields have values which specify the type of hardware and protocol addresses being utilized. For illustration HARDWARE ADDRESS TYPE has 1 while ARP is used with Ethernet and field PROTOCOL ADDRESS TYPE has 0x0800 while ARP is used along with IP. The second pair of fields HADDR LEN and PADDR LEN identifies the number of octets in a protocol address and a hardware address. Field OPERATION gives whether the message is a request or a response.
All ARP messages have fields for two address bindings. Therefore one for the recipient and another for the intended recipient that ARP calls the target.