Configuring Arp; Overview; Arp Message Format; Arp Operation - HP 6125G Configuration Manual

Configuring ARP

Overview

The Address Resolution Protocol (ARP) is used to resolve an IP address into a physical address (Ethernet
MAC address, for example).
In an Ethernet LAN, a device uses ARP to resolve the IP address of the next hop to the corresponding
MAC address.

ARP message format

ARP messages include ARP requests and ARP replies.
Numbers in the figure refer to field lengths.
Figure 1 ARP message format
ARP message fields:
Hardware type—The hardware address type. Value 1 represents Ethernet.
•
• Protocol type—The 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

If Host A and Host B are on the same subnet and Host A sends a packet to Host B, as shown in
2, the resolution process is:
Figure 1 shows the format of the ARP request/reply.
1
Figure