1
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.
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
, 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.
