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The Address Resolution Protocol (ARP) is used to resolve an IP address into an Ethernet MAC address 
(or physical address). 
In an Ethernet LAN, when a device sends data to another device, it uses ARP to translate the IP 
address of the destination device to the corresponding MAC address. 

ARP messages are classified into ARP requests and ARP replies. 

 shows the format of the 

ARP request/reply. 

ARP message format 

 

 
The following describe the fields in 

. 

z

 

Hardware type: This field specifies the hardware address type. The value “1” represents Ethernet. 

z

 

Protocol type: This field specifies the type of the protocol address to be mapped. The hexadecimal 
value “0x0800” represents IP. 

z

 

Hardware address length and protocol address length: They respectively specify the length of a 
hardware address and a protocol address, in bytes. For an Ethernet address, the value of the 
hardware address length field is "6”. For an IP(v4) address, the value of the protocol address length 
field is “4”. 

z

 

OP: Operation code. This field specifies the type of the ARP message. The value “1” represents an 
ARP request and “2” represents an ARP reply. 

z

 

Sender hardware address: This field specifies the hardware address of the device sending the 
message. 

z

 

Sender protocol address: This field specifies the protocol address of the device sending the 
message. 

z

 

Target hardware address: This field specifies the hardware address of the device the message is 
being sent to. 

z

 

Target protocol address: This field specifies the protocol address of the device the message is 
being sent to.