GATE CS 2021 Set 1 Q.18 (CS Q.8): Data Link Layer: Framing, Error Detection and Sliding Window

Consider the following two statements.

$S_1$ : Destination MAC address of an ARP reply is a broadcast address.

$S_2$ : Destination MAC address of an ARP request is a broadcast address.

Which one of the following choices is correct?

Both $S_1$ and $S_2$ are true.

$S_1$ is true and $S_2$ is false.

$S_1$ is false and $S_2$ is true.

Both $S_1$ and $S_2$ are false.

Official answer and solution

Official answer: C
From: Official GATE 2021 answer key

Solution

Recall standard Address Resolution Protocol (ARP) message delivery modes for requests and replies.

Statement $S_2$ : An ARP request is sent when a host knows the target IP address but needs to discover the corresponding target MAC address on the local network. Because the target MAC address is unknown, the ARP request frame is broadcast to all hosts using the Ethernet broadcast destination MAC address FF:FF:FF:FF:FF:FF. Hence $S_2$ is true.

Statement $S_1$ : The host that owns the requested IP address responds with an ARP reply. Because the sender already knows the MAC address of the requesting host (from the incoming ARP request header), the ARP reply is sent directly as a unicast frame addressed to the requester's MAC address. It is not broadcast. Hence $S_1$ is false.

Therefore, $S_1$ is false and $S_2$ is true, matching option C.

Final answer

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2021 Computer Science and Information Technology (CS, Set-1)
Question: 18 of 65, printed as CS Q.8
Section: Computer Science and Information Technology

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