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What is Reverse Address Resolution Protocol (RARP)?
The Reverse Address Resolution Protocol (RARP) is a network protocol described in RFC 903 that performs the opposite function of ARP. While ARP resolves IP addresses to MAC addresses, RARP resolves MAC addresses to IP addresses. This protocol is primarily used by diskless workstations and network devices that know their hardware address but need to discover their IP address during the boot process.
How RARP Works
RARP uses the same packet format as ARP but operates in reverse. A diskless system reads its hardware address from the network interface card and broadcasts a RARP request asking for its corresponding IP address. A RARP server on the network maintains a database mapping hardware addresses to IP addresses and responds with the appropriate IP address.
RARP Packet Format
RARP uses the same packet structure as ARP with key differences in the operation code field and frame type:
Operation Code − 3 for RARP request, 4 for RARP reply
EtherType Field − 0x8035 for RARP (compared to 0x0806 for ARP or 0x0800 for IP)
Hardware Address − Known parameter (source of query)
Protocol Address − Unknown parameter (target of query)
RARP vs ARP
| Feature | ARP | RARP |
|---|---|---|
| Purpose | Find MAC address from IP address | Find IP address from MAC address |
| Known Parameter | IP Address | MAC Address |
| Unknown Parameter | MAC Address | IP Address |
| Server Required | No | Yes |
| EtherType | 0x0806 | 0x8035 |
Limitations of RARP
Link-layer broadcast limitation − RARP requests cannot pass through routers, limiting functionality to local network segments
Server dependency − Requires dedicated RARP servers to maintain hardware-to-IP address mappings
Manual database management − The mapping database must be created and updated manually
Limited information − Only provides the IP address, unlike modern protocols that provide additional network configuration
Single response handling − Only processes the first RARP reply received, discarding subsequent responses
Conclusion
RARP serves a specific purpose in network bootstrapping by allowing diskless systems to discover their IP addresses using their known MAC addresses. However, its limitations led to the development of more comprehensive protocols like BOOTP and DHCP for modern network configuration.
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