ARP fundamentals: Which of the following statements about the Address Resolution Protocol (ARP) is incorrect?
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AARP allows a host to discover the physical (link-layer) address of a target on the same network, given only the target’s IP address.
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BThe sender’s IP-to-physical binding is included in every ARP broadcast, and receivers update their ARP cache before processing the packet.
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CARP hides physical addressing details, letting us assign IP addresses independently of hardware addresses.
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DAll of the above
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ENone of the above
Answer
Correct Answer: The sender’s IP-to-physical binding is included in every ARP broadcast, and receivers update their ARP cache before processing the packet.
Explanation
Introduction / Context:The Address Resolution Protocol (ARP) maps IP addresses to link-layer addresses (for example, MAC) on a local network. While its operation is straightforward, exam questions often test nuanced behaviors about broadcasting, cache updates, and protocol layering. This item asks you to spot the incorrect statement.
Given Data / Assumptions:
- ARP requests are broadcasts on the local link; ARP replies are typically unicasts.
- Each ARP packet carries the sender’s protocol and hardware addresses.
- Hosts maintain ARP caches with aging to avoid repeated broadcasts.
Concept / Approach:
Evaluate each statement against how ARP actually works. Pay attention to the words “every ARP broadcast” and “before processing,” which are red flags. ARP cache entries are usually updated upon reception/processing of a valid ARP packet, with implementation-specific safeguards (for example, gratuitous ARP handling).
Step-by-Step Solution:
Option a is true: ARP resolves an IP to a link-layer address on the same broadcast domain.Option b is problematic: although ARP requests include the sender’s binding, “every ARP broadcast” is not the whole story (replies are not broadcasts), and caches are updated upon validation/processing—not necessarily “before processing.”Option c is true conceptually: ARP abstracts hardware addressing, allowing independent IP assignment.Therefore, the incorrect statement is option b.Verification / Alternative check:
Protocol traces (for example, tcpdump) show ARP requests as broadcasts and replies as unicasts. OS implementations document when ARP caches are refreshed (often after sanity checks).
Why Other Options Are Wrong:
a: Accurate description of ARP’s purpose.
c: Correct high-level summary of ARP’s role in decoupling IP from MAC addressing.
d: “All of the above” cannot be correct because at least one statement (a or c) is true.
e: Not correct because one statement (b) is indeed incorrect.
Common Pitfalls:
Assuming all ARP packets are broadcasts, or that hosts blindly update caches without validation. Also, mixing ARP’s function with ICMP or DHCP responsibilities leads to confusion.
Final Answer:
The sender’s IP-to-physical binding is included in every ARP broadcast, and receivers update their ARP cache before processing the packet.