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Address Resolution Protocol (ARP)
373
You do not need to implement ARP — the module has ARP capability
built in, but you can manipulate and display the contents of the ARP
cache.
When the host or router knows the IP address of the next hop towards
the packet destination, the host or router translates that IP address into a
MAC address before sending the packet. To perform this translation, the
host or router first searches its
ARP cache , which is a table of IP addresses
with their corresponding MAC addresses. Each device that participates in
IP routing maintains an ARP cache. See Figure 39.
Figure 39 Example of an ARP Cache
If the IP address does not have a corresponding MAC address, the host or
router broadcasts an
ARP request packet to all the devices on the
network. The ARP request contains information about the target and
source addresses for the protocol (IP addresses). See Figure 40.
Figure 40 Example of an ARP Request Packet
When devices on the network receive this packet, they examine it. If their
address is not the target protocol address, they discard the packet. When
a device receives the packet and confirms that its IP address matches the
ARP cache
158.101.1.1
158.101.2.1
IP address MAC address
00308e3d0042
0080232b00ab
158.101.3.1
158.101.2.1
Source hardware address
?
Source protocol address
Target protocol address
Target hardware address
00802322b00ad