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Configuring ARP
Overview
ARP resolves IP addresses into MAC addresses on Ethernet networks.
ARP message format
ARP uses two types of messages: ARP request and ARP reply. Figure 1 shows the format of ARP
request/reply messages. Numbers in the figure refer to field lengths.
Figure 1 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 the type of ARP message. The value 1 represents an
ARP request, and the 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 operating mechanism
As shown in Figure 2, 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 the 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.
Then Host A sends the frame to Host B.
2. If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an ARP request.
The payload of the ARP request contains the following information:
{ Sender IP address and sender MAC address—Host A's IP address and MAC address.
{ Target IP address—Host B's IP address.
{ Target MAC address—An all-zero MAC address.
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