3Com 4800G Series, Configuration Manual

Page: 782 / 1246

782 C
HAPTER
54: ARP C
ONFIGURATION
■
Hardware address length and protocol address length: They respectively specify
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
"1" represents an ARP request and "2" represents an ARP reply.
■
Sender hardware address: This field specifies the hardware address of the
device sending the message.
■
Sender protocol address: This field specifies the protocol address of the device
sending the message.
■
Target hardware address: This field specifies the hardware address of the device
the message is being sent to.
■
Target protocol address: This field specifies the protocol address of the device
the message is being sent to.
ARP Address Resolution
Process
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
Target IP
address
192 .168 .1 .1
Target IP
address
192 .168 .1 .2
Host A
192.168.1.1
0002- 6779-0 f4c Host B
192.168.1.2
00a 0-2470 -febd
Target MAC
address
0000 -0000- 0000
Sender MAC
address
00a 0-2470 -febd
Target MAC
address
0002 -6779 -0f4c
Sender IP
address
192.168 .1.1
Sender MAC
address
0002 -6779 -0f4c
Sender IP
address
192.168 .1.2