In IP networking, what is Proxy ARP and what problem does it solve on a local network segment?

Introduction / Context:
Address Resolution Protocol, or ARP, is used on IPv4 networks to map IP addresses to MAC addresses on a local link. In some cases, it is useful for a router to respond to ARP requests for an IP address that does not actually belong to the local link, effectively pretending that a remote host is local. This behaviour is called Proxy ARP. Understanding Proxy ARP helps when dealing with complex subnetting and legacy network configurations.

Given Data / Assumptions:

- We are operating on a local Ethernet or similar broadcast segment where ARP requests are sent.- There is at least one router capable of forwarding packets to remote subnets.- Hosts may be configured with addresses that are not in the same subnet as the local network.

Concept / Approach:
Proxy ARP allows a router to reply to an ARP request for an IP address that resides on another network, using its own MAC address. When a local host sends an ARP request for that remote IP, the router responds with its MAC address. The host then sends packets to the router, which forwards them to the actual destination. To the host, it appears as though the remote host is on the same local subnet. This can be used to smooth over misconfigured subnet masks or to connect isolated hosts without changing their configuration. However, Proxy ARP can also create confusion and security concerns if misused.

Step-by-Step Solution:
Step 1: Recall that normal ARP is used by a host to find the MAC address associated with an IP address on the same network segment.Step 2: In a network that uses Proxy ARP, the router listens for ARP requests for IP addresses that belong to remote networks it can reach.Step 3: When such a request is heard, the router replies with its own MAC address, effectively saying that the host can reach that IP through the router.Step 4: The local host updates its ARP cache to map the remote IP address to the router MAC address and sends packets to that MAC.Step 5: The router then forwards the packets to the real destination network using normal routing.Step 6: Option A describes this exact process: a router answers ARP on behalf of another host, making it appear local.Step 7: Options B, C, D, and E assign encryption, translation, address assignment, or DNS broadcast functions to Proxy ARP, none of which match its true purpose.

Verification / Alternative check:
Networking references describe Proxy ARP as a way for a router to act as a proxy for ARP requests, and configuration guides show commands on routers that enable or disable Proxy ARP on interfaces. Example scenarios include connecting dial in clients directly to a LAN or supporting hosts with incorrect subnet masks. These documents clarify that Proxy ARP does not encrypt packets, assign addresses, or translate between IP versions, confirming that option A is the correct description.

Why Other Options Are Wrong:
Encryption of ARP is rare and would be handled by link layer security mechanisms, not by Proxy ARP. Translating between IPv4 and IPv6 uses techniques such as NAT64 and does not involve ARP proxying. Dynamic IP assignment is done by DHCP or related protocols, not ARP. DNS query broadcasting is part of DNS resolver behaviour and is unrelated to ARP at layer 2.

Common Pitfalls:
Students sometimes think that any advanced ARP behaviour must involve security or encryption, but Proxy ARP is more about convenience and compatibility. Another pitfall is to confuse Proxy ARP with NAT. While both involve routers acting on behalf of other hosts, Proxy ARP operates at layer 2 for ARP messages, while NAT operates at layer 3 and 4 modifying IP addresses and ports. Keeping this layering difference in mind can help avoid confusion.

Final Answer:
Proxy ARP is a technique where a router answers ARP requests on behalf of another host, making that remote host appear to be on the same local network segment, as stated in option A.