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4-8 CHAPTER 4: ROUTING WITH IP TECHNOLOGY
protocol. The two key elements of the ARP request are the target and
source addresses for both the hardware (MAC addresses) and the
protocol (IP addresses). See Figure 4-7.
Figure 4-7 Example of an ARP Request Packet
When the devices on the network receive this packet, they examine it,
and 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 target protocol address, this device places its MAC
address in the target hardware address field and sends the packet back
to the source hardware address. When the originating host or router
receives the ARP reply, it places the new MAC address in its ARP cache
next to the corresponding IP address. See Figure 4-8.
Figure 4-8 Example of ARP Cache Updated with ARP Reply
Once the MAC address is known, the host or router can send the packet
directly to the next hop.
158.101.2.15
158.101.2.1
Source hardware address
?
ARP Request
Source protocol address
Target protocol address
Target hardware address
00802322b00ad
ARP Cache
158.101.1.1
158.101.2.1
158.101.3.1
IP Address MAC Address
00308e3d0042
0080232b00ab
0134650f3000