Question
Suppose that in an IP-over-Ethernet network, a machine X wishes to find the MAC address of another machine Y in its subnet. Which one of the following techniques can be used for this?
Official answer and solution
- Official answer
- C
- From
- Official GATE 2019 answer key
Solution
Recall the Address Resolution Protocol (ARP) mechanism at the Data Link layer.
When machine wants to communicate with machine on the same local network subnet, knows 's IP address but needs 's physical (MAC) address to construct the Ethernet frame.
To resolve this, broadcasts an ARP Request packet over the local Ethernet network:
- The Ethernet frame's destination MAC address is set to the broadcast address
FF:FF:FF:FF:FF:FF. - Every machine on the local subnet receives and inspects the frame.
- The machine whose IP address matches the target IP in the ARP payload (machine ) replies with a unicast ARP Reply containing its MAC address.
Therefore, sends an ARP request packet with broadcast MAC address in its local subnet (Option C).
C
Thinking ARP uses a broadcast IP address or goes through the default gateway for local subnet targets.
The answer is the one in the official GATE answer key. The solution is this site's own explanation.
Source
- Paper
- GATE 2019 Computer Science and Information Technology (CS)
- Question
- 39 of 65, printed as CS Q.29
- Section
- Computer Science and Information Technology
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