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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Network Interface / Internet "Layer Connection" Protocols

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TCP/IP Address Resolution For IP Version 6
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TCP/IP Internet Layer (OSI Network Layer) Protocols
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Reverse Address Resolution and the TCP/IP Reverse Address Resolution Protocol (RARP)
(Page 2 of 4)

The Reverse Address Resolution Protocol (RARP)

The first method devised to address the bootstrapping problem in TCP/IP was the backwards use of ARP I mentioned above. This technique was formalized in RFC 903, A Reverse Address Resolution Protocol (RARP), published in 1984. Where ARP allows device A to say “I am device A and I have device B's IP address, device B please tell me your hardware address”, RARP is used by device A to say “I am device A and I am sending this broadcast using my hardware address, can someone please tell me my IP address?”.The two-step operation of RARP is illustrated in Figure 52.


Figure 52: Operation of the Reverse Address Resolution Protocol (RARP)

RARP, as the name suggests, works like ARP but in reverse, so this diagram is similar to Figure 47. Here, instead of Device A providing the IP address of another device and asking for its hardware address, it is providing its own hardware address and asking for an IP address it can use. The answer, in this case, is provided by Device D, which is serving as an RARP server for this network.

 


The next question then is: who knows A's IP address if device A doesn't? The answer is that a special RARP server must be configured to listen for RARP requests and issue replies to them. Each physical network where RARP is in use must have RARP software running on at least one machine.

RARP is not only very similar to ARP, it basically is ARP. What I mean by this is that RFC 903 doesn't define a whole new protocol from scratch, it just describes a new method for using ARP to perform the opposite of its normal function. RARP uses ARP messages in exactly the same format as ARP, but uses different opcodes to accomplish its reverse function. Just as in ARP, a request and reply are used in an exchange. The meaning of the address fields is the same too: the sender is the device transmitting a message while the target is the one receiving it.

Key Concept: The Reverse Address Resolution Protocol (RARP) is the earliest and simplest protocol designed to allow a device to obtain an IP address for use on a TCP/IP network. It is based directly on ARP and works in basically the same way, but in reverse: a device sends a request containing its hardware address and a device set up as an RARP server responds back with the device’s assigned IP address.



Previous Topic/Section
TCP/IP Address Resolution For IP Version 6
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34
Next Page
TCP/IP Internet Layer (OSI Network Layer) Protocols
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