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 216 shows the format of the ARP request/reply messages. Numbers in the figure refer to field lengths.

Figure 216 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 operating mechanism

As shown in Figure 217, 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. Then Host A sends the frame to Host B.

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