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9.3 | |
Obtaining an IP address | |
| 9.3.3 | |
RARP IP address assignment |
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Reverse Address Resolution Protocol (RARP)
associates a known MAC addresses with an IP addresses. This
association allows network
devices to encapsulate data before sending the data out on the
network. A network device, such as a diskless workstation, might know
its MAC address but not its IP address. RARP allows the device to make
a request to learn its IP address. Devices using RARP require that a
RARP server be present on the network to answer RARP requests.
Consider an example where a source device wants to send data to another device. In this example, the source device knows its own MAC address but is unable to locate its own IP address in the ARP table. The source device must include both its MAC address and IP address in order for the destination device to retrieve data, pass it to higher layers of the OSI model, and respond to the originating device. Therefore, the source initiates a process called a RARP request. This request helps the source device detect its own IP address. RARP requests are broadcast onto the LAN and are responded to by the RARP server which is usually a router. RARP uses the same packet format as
ARP. However, in a RARP request, the MAC headers and "operation code"
are different from an ARP request.
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The RARP packet format contains
places for MAC addresses of both the destination and source devices.
The source IP address field is empty. The broadcast goes to all
devices on the network. Therefore, the destination MAC address will be
set to all binary 1s. Workstations running RARP have codes in ROM that
direct them to start the RARP process. A step-by-step layout of the
RARP process is illustrated in Figures
through
.
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