ARP vs RARP (Address Resolution Protocol vs Reverse Address Resolution Protocol)

Both are network-layer support protocols that map between a device's IP (logical) address and its MAC (physical) address, but they perform exactly opposite mappings.

1. Basic definitions

• ARP resolves a known IP address → unknown MAC address. When a host knows the destination's IP (e.g. from DNS) but must build an Ethernet frame, it broadcasts an ARP request on the LAN; the owner of that IP replies with its MAC, which is then cached in the ARP table.
• RARP resolves a known MAC address → unknown IP address. A diskless station that knows only its own MAC (read from the NIC) broadcasts a RARP request asking "what is my IP?"; a RARP server on the LAN replies with the assignment.

2. Comparison table

Basis	ARP	RARP
Direction of mapping	IP address → MAC address	MAC address → IP address
Who uses it	Any host/router that needs to frame a packet	Diskless/thin client that boots without local IP configuration
Protocol layer	Network layer, just above the data link layer (operates over Ethernet)	Network layer, just above the data link layer
Message types	ARP request (broadcast) / ARP reply (unicast), also reverse ARP & gratuitous ARP	RARP request (broadcast) / RARP reply (unicast)
Server needed	No server required — any host answers for itself	Dedicated RARP server required on every LAN
Configuration stored	In each host's ARP cache (dynamic + static entries)	Centralised on the RARP server (file/database)
Message format	Same 28-byte packet for both request and reply (hardware type, protocol type, hlen/plen, opcode, SHA/SPA/THA/TPA)	Same format as ARP with opcode 3/4
Broadcast used	Only when the MAC for a target IP is not in the cache	Always, since the client has no IP and cannot use IP-based services
Limitations	Cannot cross routers (broadcast domain); prone to ARP spoofing/poisoning	LAN-limited, servers must be on the same segment; largely obsolete
Replaced/superseded by	Still fundamental to IPv4 (proxy ARP, gratuitous ARP)	BOOTP and DHCP, which also supply subnet mask, default gateway, DNS, etc.
Direction of query	"Who has 192.168.1.5? Tell 192.168.1.10"	"I am 00:AA:BB:CC:DD:EE; what is my IP?"

3. Operational flow

ARP (IP → MAC):

1. Host A wants to send to 192.168.1.5 and checks its ARP cache — entry missing.
2. A broadcasts an ARP request: "Who has 192.168.1.5?".
3. Every host on the LAN receives it; only the owner of 192.168.1.5 replies unicast: "192.168.1.5 is at 00:11:22:33:44:55".
4. A stores the pair in its ARP cache (typically 15–45 min TTL) and now frames the actual data.

RARP (MAC → IP):

1. Diskless machine B reads its MAC 00:AA:BB:CC:DD:EE at boot.
2. B broadcasts a RARP request: "My MAC is 00:AA:BB:CC:DD:EE, what is my IP?".
3. The RARP server on that LAN looks up the MAC in its table and unicasts back the assigned IP (and the server's own IP).
4. B configures its network interface and continues booting.

4. Key differences in one line

ARP lets an already-configured host reach other hosts on the link by discovering hardware addresses for known IP addresses; RARP lets an unconfigured host discover its own IP from its hardware address — a capability now provided by BOOTP/DHCP.