not its IP address. Devices using RARP require that a RARP server be present on the network to answer RARP requests. Let's look at an example where a source device wants to send data to another device. In our example the source knows its own MAC address, but is unable to locate its own IP address in its ARP table. In order for the destination device to retrieve the data, pass it to higher layers of the OSI model, and respond to the originating device, the source must include both its MAC address and IP address. Therefore, the source initiates a process called a RARP request, which helps it detect its own IP address. The device builds a RARP request packet and sends it out on the network. To ensure that all devices see the RARP request on the network, it uses a broadcast IP address. RARP uses the same packet format as ARP. But in a RARP request, the MAC headers, IP headers, and "operation code" are different from an ARP request. The RARP packet format contains places for MAC addresses of both destination and source. The source IP address field is empty. The broadcast goes to all devices on the network; therefore the destination IP address will be set to all binary 1s. Workstations running RARP have codes in ROM that direct them to start the RARP process, and locate the RARP server. BOOTstrap Protocol (BOOTP) A device uses BOOTstrap protocol (BOOTP) when it starts up, to obtain an IP address. BOOTP uses UDP to carry messages; the UDP message is encapsulated in an IP datagram. A computer uses BOOTP to send a broadcast IP datagram (using a destination IP address of all 1s - 255.255.255.255). A BOOTP server receives the broadcast and then sends a broadcast. The client receives a datagram and checks the MAC address. If it finds its own MAC address in the destination address field, then it takes the IP address in that datagram. Like RARP, BOOTP operates in a client-server environment, and only requires a single packet exchange. However, unli...
