Which of the following occurs when a device receives an ARP request for its own IP address?

Which of the following occurs when a device receives an ARP request for its own IP address?

A) It ignores the request
B) It sends an ARP reply with its MAC address
C) It broadcasts an ARP request
D) It sends an ARP reply with its IP address

Answer:

B) It sends an ARP reply with its MAC address

Option A: It ignores the request

Explanation:

In a properly functioning network, when a device receives an ARP request, it does not ignore the request if the IP address in the request matches its own. Ignoring the request would mean the requesting device could not obtain the necessary MAC address to communicate, leading to a failure in communication.

However, there are certain scenarios where a device might ignore an ARP request:

1. Security Configurations: In some networks, devices may be configured to ignore ARP requests for security reasons. For example, in a network employing ARP security measures such as Dynamic ARP Inspection (DAI), if an ARP request is considered suspicious or does not comply with the network’s ARP policy, the device may ignore it to prevent ARP spoofing or poisoning attacks.
2. Mismatch or Misconfiguration: If there is a network misconfiguration or if the IP address in the ARP request does not match any device on the network, the ARP request would be ignored by all devices. In such cases, the device has no obligation to respond because the request is not relevant to it.
3. Network Congestion or Overload: In extremely congested or overloaded networks, devices may fail to process ARP requests in a timely manner, which could appear as if the requests are being ignored. However, this situation is typically an exception rather than the rule.

Ignoring an ARP request in normal circumstances would disrupt network communication and is generally not the behavior of a properly configured device.

Option B: It sends an ARP reply with its MAC address

Explanation:

This is the correct answer. When a device receives an ARP request for its own IP address, it sends an ARP reply containing its MAC address directly to the requester. This response allows the requesting device to map the IP address to the corresponding MAC address, enabling successful data transmission between the two devices.

Here’s a step-by-step explanation of the process:

1. ARP Request Broadcast: The device that needs to communicate with another device sends out an ARP request as a broadcast to the entire local network. This request asks, “Who has IP address X? Tell me your MAC address.”
2. Reception by the Target Device: The device on the network that has the matching IP address receives the ARP request.
3. ARP Reply Unicast: The target device then prepares an ARP reply that contains its MAC address. This reply is sent directly to the requesting device, not as a broadcast but as a unicast message, meaning it is sent specifically to the MAC address of the requester.
4. ARP Table Update: Upon receiving the ARP reply, the requesting device updates its ARP table with the MAC address corresponding to the IP address. This allows the device to send data directly to the target device using its MAC address.

This process is fundamental to enabling communication on a local network, where devices must resolve IP addresses to MAC addresses to transmit data effectively at the data link layer (Layer 2) of the OSI model.

Option C: It broadcasts an ARP request

Explanation:

This option is incorrect in the context of the original question. When a device receives an ARP request that matches its own IP address, it does not respond by broadcasting another ARP request. Instead, it sends an ARP reply directly to the requesting device, as explained in Option B.

However, let’s explore the scenario where broadcasting an ARP request is appropriate:

1. Original ARP Request: When a device needs to determine the MAC address associated with a particular IP address, it will broadcast an ARP request to all devices on the local network. This broadcast message is sent to the broadcast MAC address (FF:FF:FF:FF:FF

   ), ensuring that all devices on the local network receive it.

2. Purpose of Broadcasting: The reason for broadcasting the ARP request is that the requesting device does not know the MAC address of the target device. By broadcasting, it ensures that the target device, which has the corresponding IP address, will receive the request and can respond with its MAC address.
3. Result of Broadcasting: Once the target device responds with its MAC address, the ARP process is completed, and the requesting device updates its ARP table.

Thus, broadcasting an ARP request is part of the process of discovering the MAC address of a device, but it is not the correct response when a device receives an ARP request for its own IP address.

Option D: It sends an ARP reply with its IP address

Explanation:

This option is also incorrect. The ARP protocol is designed to map IP addresses to MAC addresses, not to resolve IP addresses back to IP addresses. When a device receives an ARP request for its own IP address, it is expected to respond with its MAC address, not its IP address.

Here’s why sending an ARP reply with its IP address would not make sense:

1. Purpose of ARP: The primary function of ARP is to resolve IP addresses to MAC addresses. The ARP request is essentially asking, “What is the MAC address associated with this IP address?” The reply needs to provide the MAC address to fulfill this request.
2. Redundancy of IP Information: The requesting device already knows the IP address; that’s why it sent the ARP request in the first place. If the device responded with only its IP address, it wouldn’t provide any new information that the requester needs to send data on the local network.
3. Correct Response: The correct response to an ARP request is to send the MAC address associated with the requested IP address. This allows the requesting device to complete the ARP process and establish communication.

Sending an ARP reply with the IP address would be redundant and would not serve the purpose of the ARP protocol. The ARP process is specifically designed to map IP addresses to MAC addresses, enabling devices to communicate on a local network efficiently.

Conclusion

The ARP process is fundamental to the operation of a local network, enabling devices to resolve IP addresses to MAC addresses for communication at the data link layer. Understanding how devices respond to ARP requests is crucial for network troubleshooting and security.

When a device receives an ARP request for its own IP address, it correctly responds by sending an ARP reply with its MAC address (Option B). This response allows the requesting device to map the IP address to a MAC address and successfully communicate with the target device.

Other options, such as ignoring the request, broadcasting another ARP request, or sending an ARP reply with the IP address, are incorrect in this context and would either disrupt communication or fail to fulfill the purpose of ARP. Understanding these nuances is vital for anyone working in networking or preparing for certifications like the CCNA.