GATE CS 2019 Q29 — Concept of Layering: OSI and TCP/IP Protocol Stacks

To find the MAC address of machine $Y$ in the same subnet, machine $X$ uses the Address Resolution Protocol (ARP). The process works as follows:

1. Direct Communication: Since machine $Y$ is in the same subnet as $X$, the local gateway is not involved. $X$ can communicate with $Y$ directly. This eliminates Options A and B.
2. Layer 2 Broadcast: Because $X$ does not know $Y$'s MAC address, it must broadcast the ARP request so that all hosts in the local subnet receive it. This is achieved by encapsulating the ARP request in an Ethernet frame with the destination MAC address set to the broadcast MAC address (FF:FF:FF:FF:FF:FF). Thus, Option C is correct.
3. No IP Broadcast: ARP packets are encapsulated directly inside Ethernet frames (Data Link Layer) and do not use an IP header. The target IP address field inside the ARP payload is set to $Y$'s specific unicast IP address, not a broadcast IP address. This eliminates Option D.

Therefore, $X$ sends an ARP request packet with the broadcast MAC address in its local subnet.