In TCP/IP networks, what is the difference between ARP (Address Resolution Protocol) and RARP (Reverse Address Resolution Protocol)?

Introduction / Context:
Address Resolution Protocol (ARP) and Reverse Address Resolution Protocol (RARP) are two related but opposite mechanisms used in older TCP/IP networks. ARP is still widely used in IPv4 networks today, while RARP is largely historical and has been replaced by more modern mechanisms like BOOTP and DHCP. Interview questions often ask for the difference between ARP and RARP to test understanding of how IP and MAC addresses are linked.

Given Data / Assumptions:

• We are working with IPv4 on Ethernet or similar link layers.
• Devices need to send frames using MAC addresses on the local network.
• Some older or diskless hosts might not know their IP address at boot time.
• We are focusing on the direction of mapping between IP and MAC addresses.

Concept / Approach:
ARP is used by a host that knows an IPv4 address but needs the corresponding MAC address to send a frame on the LAN. It broadcasts an ARP request asking "Who has this IP address? Tell me your MAC," and the owner responds with an ARP reply containing its MAC. RARP performs the reverse lookup: a host that knows its own MAC address but does not know its IP sends a RARP request, and a RARP server replies with the correct IP for that MAC. This allowed diskless workstations to discover their IP addresses at boot time in early networks.

Step-by-Step Solution:
Step 1: Define ARP as Address Resolution Protocol used to map a known IP address to a MAC address. Step 2: Explain that ARP requests are broadcast, and the host with the matching IP replies with its MAC address. Step 3: Define RARP as Reverse Address Resolution Protocol used to map a known MAC address to an IP address, typically via a RARP server. Step 4: Note that RARP requests are also broadcast, and the server responds with the appropriate IP for that MAC. Step 5: Emphasize that ARP is still widely used in IPv4, while RARP has mostly been replaced by BOOTP and DHCP.

Verification / Alternative check:
Packet captures on IPv4 networks show frequent ARP traffic, with requests like "Who has 192.168.1.1? Tell 192.168.1.10." The ARP reply contains the MAC address of 192.168.1.1. Historical documentation of RARP describes how a client knowing only its MAC would send a RARP request, and a server with preconfigured mappings would answer with an IP address. Modern systems instead use DHCP broadcasts to obtain IP configuration, confirming that RARP is conceptually the reverse of ARP but largely obsolete in practice.

Why Other Options Are Wrong:
Option B incorrectly introduces hostnames, which are resolved via DNS, not ARP or RARP. Option C wrongly associates ARP solely with IPv6; IPv6 uses Neighbor Discovery instead of ARP, and ARP is specifically for IPv4. Option D incorrectly claims the protocols are identical, ignoring their opposite mapping directions and different historical usage.

Common Pitfalls:
Students sometimes confuse ARP with DNS, thinking ARP maps hostnames to IP addresses. Another pitfall is assuming RARP is still commonly used in modern networks; in reality, DHCP provides much more flexible configuration. Remembering that ARP maps IP to MAC and RARP maps MAC to IP helps keep the concepts clear during troubleshooting and in theory questions.

Final Answer:
ARP and RARP differ in direction: ARP resolves a known IP address to a MAC address, while RARP resolves a known MAC address to an IP address.