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Appendix C     Packet Header Formats

Different fields in the ARP packet header are described in Table C 4.

Table C 4 ARP Packet Header Fields

Field

Explanation

HW Address Type

The HW Address type is a 16 bit long field and it shows the type of hard 

ware. Since most of LANs are Ethernet based, its value is 1. For IEEE 802 

networks, its value is 6. For IPSec tunnel, the value is 31.

Protocol Address Type

The protocol address type shows the protocol used in the network layer. 

The value of this field is 0x800 for IP.

HW Addr Len

This field shows the length of the hardware address in number of bytes. 

This field is 8 bits long.

Proto Addr Length

This field shows the length of the protocol address. This field is also 8 bits 

long.

Operation or Opcode

This field is 16 bits long and is used for the type of ARP packet. A value 

of 1 indicates a request packet and a value of 2 indicates a reply packet.

Source hardware address

This is a 48 bit long field in the case of Ethernet. However its length is 

variable.

Source protocol address

This is a 32 bit field in the case of IPv4 packets. However its length is vari 

able.

Target hardware address

This is 48 bits long in  Ethernet and its length is variable.

Target protocol address

This is 32 bits in the case of IPv4 and its length is variable.