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Configuring ARP
Overview
ARP resolves an IP address into a physical address, such as an Ethernet MAC address.
On an Ethernet LAN, a device uses ARP to get the MAC address of the target device for a packet.
ARP message format
ARP uses two types of messages, ARP request and ARP reply. Figure 206 shows the format of the ARP
request/reply. Numbers in the figure refer to field lengths.
Figure 206 ARP message format
• Hardware type—Hardware address type. The value 1 represents Ethernet.
• Protocol type—Type of the protocol address to be mapped. The hexadecimal value 0x0800
represents IP.
• Hardware address length and protocol address length—Length, in bytes, of a hardware address
and a protocol address. For an Ethernet address, the value of the hardware address length field is
6. For an IPv4 address, the value of the protocol address length field is 4.
• OP—Operation code, which describes type of the ARP message. Value 1 represents an ARP request,
and value 2 represents an ARP reply.
• Sender hardware address—Hardware address of the device sending the message.
• Sender protocol address—Protocol address of the device sending the message.
• Target hardware address—Hardware address of the device to which the message is being sent.
• Target protocol address—Protocol address of the device to which the message is being sent.
ARP operation mechanism
As shown in Figure 207, Host A and Host B are on the same subnet. Host A sends a packet to Host B as
follows:
1. Host A looks through its ARP table for an ARP entry for Host B. If one entry is found, Host A uses
the MAC address in the entry to encapsulate the IP packet into a data link layer frame and sends
the frame to Host B.
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