3Com Switch 4800G 24-Port Configuration Manual page 782

782 C 54: ARP C
HAPTER
ARP Address Resolution
Process
ONFIGURATION
Hardware address length and protocol address length: They respectively specify
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the length of a hardware address and a protocol address, in bytes. For an
Ethernet address, the value of the hardware address length field is "6". For an
IP(v4) address, the value of the protocol address length field is "4".
OP: Operation code. This field specifies the type of ARP message. The value
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"1" represents an ARP request and "2" represents an ARP reply.
Sender hardware address: This field specifies the hardware address of the
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device sending the message.
Sender protocol address: This field specifies the protocol address of the device
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sending the message.
Target hardware address: This field specifies the hardware address of the device
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the message is being sent to.
Target protocol address: This field specifies the protocol address of the device
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the message is being sent to.
Suppose that Host A and Host B are on the same subnet and that Host A sends a
message to Host B, as show in Figure 236. The resolution process is as follows:
1 Host A looks in its ARP mapping table to see whether there is an ARP entry for
Host B. If Host A finds it, 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.
2 If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an
ARP request, in which the source IP address and source MAC address are
respectively the IP address and MAC address of Host A and the destination IP
address and MAC address are respectively the IP address of Host B and an all-zero
MAC address. Because the ARP request is sent in broadcast mode, all hosts on this
subnet can receive the request, but only the requested host (namely, Host B) will
process the request.
3 Host B compares its own IP address with the destination IP address in the ARP
request. If they are the same, Host B saves the source IP address and source MAC
address into its ARP mapping table, encapsulates its MAC address into an ARP
reply, and unicasts the reply to Host A.
4 After receiving the ARP reply, Host A adds the MAC address of Host B into its ARP
mapping table for subsequent packet forwarding. Meanwhile, Host A
encapsulates the IP packet and sends it out.
Figure 236 ARP address resolution process
Host A
192.168.1.1
0002- 6779-0 f4c
Sender MAC
0002 -6779 -0f4c
Sender MAC
00a 0-2470 -febd
Sender IP
address address
192.168 .1.1
Sender IP
address address
192.168 .1.2
00a 0-2470 -febd
Target MAC
Target IP
address
address
0000 -0000- 0000
192 .168 .1 .2
Target MAC
Target IP
address
address
0002 -6779 -0f4c
192 .168 .1 .1
Host B
192.168.1.2