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1-1

1 ARP Configuration

Introduction to ARP

ARP Function

Address Resolution Protocol (ARP) is used to resolve an IP address into a data link layer address.

An IP address is the address of a host at the network layer. To send a network layer packet to a

destination host, the device must know the data link layer address (MAC address, for example) of the

destination host or the next hop. To this end, the IP address must be resolved into the corresponding

data link layer address.

Unless otherwise stated, a data link layer address in this chapter refers to a 48-bit Ethernet MAC

address.

ARP Message Format

ARP messages are classified as ARP request messages and ARP reply messages. Figure 1-1

illustrates the format of these two types of ARP messages.

z As for an ARP request, all the fields except the hardware address of the receiver field are set. The

hardware address of the receiver is what the sender requests for.

z As for an ARP reply, all the fields are set.

Figure 1-1 ARP message format

Hardware type (16 bits)

Protocol type (16 bits)

Length of hardware address

Length of protocol address

Operator (16 bits)

Hardware address of the sender

IP address of the sender

Hardware address of the receiver

IP address of the receiver

Hardware type (16 bits)Hardware type (16 bits)