Suppose that in an IP-over-Ethernet network, a machine X wishes to find the…
2019
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?
- A.
X sends an ARP request packet to the local gateway’s IP address which then finds the MAC address of Y and sends to X
- B.
X sends an ARP request packet to the local gateway’s MAC address which then finds the MAC address of Y and sends to X
- C.
X sends an ARP request packet with broadcast MAC address in its local subnet
- D.
X sends an ARP request packet with broadcast IP address in its local subnet
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Correct answer: C
Key idea: when a host on the same subnet needs another host’s MAC, it uses ARP with an Ethernet broadcast.
Step 1: X sends an ARP request frame on the local Ethernet with the destination MAC set to the broadcast address ff:ff:ff:ff:ff:ff. The ARP payload includes Y’s IP as the target IP.
Step 2: Every host on the subnet receives the frame; only the host that owns the target IP (Y) replies with a unicast ARP reply containing its MAC address.
Step 3: X receives the reply and stores Y’s MAC in its ARP table for future use.
Note: A gateway/router is only involved if Y is outside X’s subnet; for local hosts the gateway is not used to resolve MAC addresses.