In TCP or IP networking, what is the main difference between ARP and RARP?

Introduction / Context:
In local area networks that use TCP or IP, devices must map between logical IP addresses and physical hardware addresses such as MAC addresses. Two early protocols designed for this purpose are the Address Resolution Protocol and the Reverse Address Resolution Protocol. Exam questions often ask for the difference between these two closely named protocols. Understanding which direction each protocol maps addresses is the key to answering correctly.

Given Data / Assumptions:

• The network stack in question is based on TCP or IP.
• Devices on a local network have IP addresses and MAC addresses.
• ARP and RARP are used for address resolution in opposite directions.
• The question asks for the main difference between ARP and RARP.

Concept / Approach:
ARP is used when a host knows the destination IP address but needs to discover the corresponding hardware address on the local network in order to send a frame. It broadcasts a request asking which device owns a given IP address and receives a reply with the MAC address. RARP operates in the opposite direction. It was designed so that a diskless workstation that knows only its own MAC address can query a RARP server to obtain its IP address at boot time. Thus, ARP maps IP to MAC, while RARP maps MAC to IP.

Step-by-Step Solution:
Step 1: Recall that sending an Ethernet frame requires the sender to know the destination MAC address.Step 2: Understand that when a host has an IP packet for a local destination but does not know the MAC address, it uses ARP to ask for the MAC corresponding to that IP address.Step 3: Remember that some early hosts, such as diskless workstations, booted without knowing their IP addresses but did know their MAC addresses.Step 4: These hosts could use RARP to send a request containing their MAC address to a server, which responded with the correct IP address.Step 5: Conclude that ARP resolves IP to MAC, while RARP resolves MAC to IP.

Verification / Alternative check:
Network documentation and protocol descriptions show ARP packets containing a target protocol address (IP) and a resulting hardware address (MAC). In contrast, RARP packets carry a hardware address as input and request a protocol address as output. Modern networks often replace RARP with more flexible protocols such as BOOTP or DHCP, but the conceptual mapping directions remain standard exam knowledge. This confirms the distinction presented in the question.

Why Other Options Are Wrong:
Option B is incorrect because both ARP and RARP were defined for use on Ethernet style networks, and neither is limited specifically to wireless or wired usage in the way described. Option C is wrong because ARP and RARP operate near the boundary between the data link and network layers, not at the routing or application layers. Option D is incorrect because the two protocols are not identical; they perform complementary mappings in opposite directions.

Common Pitfalls:
A frequent mistake is to remember that ARP and RARP are related to address resolution but forget which one maps in which direction. Another pitfall is to assume that RARP is still widely used; while it has been largely replaced by more advanced protocols, it remains important historically and in exam questions. A simple memory aid is that ARP starts from the IP address and finds the MAC, while RARP reverses that relationship.

Final Answer:
The main difference is that ARP resolves a known IP address to a hardware or MAC address, while RARP resolves a known hardware or MAC address to an IP address.