1-2
z Sender protocol address: This field specifies the protocol address of the device sending the
message.
z Target hardware address: This field specifies the hardware address of the device the message is
being sent to.
z Target protocol address: This field specifies the protocol address of the device the message is
being sent to.
ARP Operation
Suppose that Host A and Host B are on the same subnet and Host A sends a packet to Host B, as
shown in
Figure 1-2. The resolution process is as follows:
1) Host A looks into its ARP table to see whether there is an ARP entry for Host B. If yes, 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 sender IP address and the sender MAC address are the IP address and the MAC
address of Host A respectively, and the target IP address and the target MAC address are the IP
address of Host B and an all-zero MAC address respectively. Because the ARP request is a
broadcast, all hosts on this subnet can receive the request, but only the requested host (namely,
Host B) will respond to 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 in its ARP 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 to its ARP table. Meanwhile,
Host A encapsulates the IP packet and sends it out.
Figure 1-2 ARP address resolution process
If Host A is not on the same subnet with Host B, Host A first sends an ARP request to the gateway. The
target IP address in the ARP request is the IP address of the gateway. After obtaining the MAC address
of the gateway from an ARP reply, Host A sends the packet to the gateway. If the gateway maintains the
ARP entry of Host B, it forwards the packet to Host B directly; if not, it broadcasts an ARP request, in
which the target IP address is the IP address of Host B. After obtaining the MAC address of Host B, the
gateway sends the packet to Host B.
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