Which mechanism allows a router to respond to an ARP request on behalf of a remote host, effectively providing its own MAC address instead?

Introduction / Context:
Address Resolution Protocol is used on broadcast networks to map IP addresses to MAC addresses. Sometimes, a host may attempt to reach another IP address that is not on the same subnet. In certain legacy or special configurations, routers can assist by answering ARP requests on behalf of remote hosts, providing their own MAC address so that traffic is forwarded correctly.

Given Data / Assumptions:

• A router is connected to multiple networks or subnets.
• A host sends an ARP request for an IP address that is not on its local subnet.
• The router has a route to the requested IP address on a different network.
• The question asks which mechanism lets the router answer the ARP request for that remote host.

Concept / Approach:
Proxy ARP enables a router to reply to ARP requests intended for a host that is not directly reachable on the local network. The router answers with its own MAC address, effectively telling the requesting host to send the traffic to the router. The router then forwards the packet toward the true destination using its routing table. This provides a form of transparent subnetting, though it is generally avoided in modern designs because it can introduce confusion.

Step-by-Step Solution:
1. A host sends an ARP request for a destination IP that lies on a remote subnet.2. The router recognizes that it has a route to that IP and that Proxy ARP is enabled on the interface receiving the ARP.3. Instead of ignoring the ARP, the router replies with its own MAC address corresponding to that IP.4. The host updates its ARP cache, associating the destination IP with the router's MAC.5. The host then sends packets for that destination to the router, which forwards them appropriately.

Verification / Alternative check:
Network documentation and configuration guides describe Proxy ARP as this specific feature. Packet captures on an interface with Proxy ARP enabled will show the router replying to ARP requests for IP addresses that are not assigned to the local segment, confirming the behavior.

Why Other Options Are Wrong:
Option A, Gateway DP, is not a standard networking mechanism. Option B, Reverse ARP, is used by a host to learn its own IP address from a server based on its MAC, not to answer ARP for remote hosts. Option D, Inverse ARP, is used in Frame Relay and similar technologies to discover network layer addresses corresponding to known virtual circuit identifiers, not to respond to Ethernet ARP queries.

Common Pitfalls:
Some learners confuse Proxy ARP with NAT or routing in general. Proxy ARP works specifically at the ARP layer by manipulating MAC address responses. It does not change IP addresses or provide broad routing functionality by itself; it simply allows a router to masquerade as another host in ARP responses.

Final Answer:
Proxy ARP