3Com 2500 manual 7 Example of an ARP Request Packet

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.

ARP Request

00802322b00ad Source hardware address

158.101.2.1

Source protocol address

?

Target hardware address

158.101.2.15 Target protocol address

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.

ARP Cache

IP Address MAC Address

158.101.1.1 00308e3d0042

158.101.2.1 0080232b00ab

158.101.3.1 0134650f3000

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.