Reverse Address resolution protocol (RARP), Bootstrap Protocol (BOOTP)
These protocols are nowhere used nowadays. They are replaced by DHCP (Dynamic Host Configuration Protocol). Reverse ARP assigns IP address to a known MAC address. Suppose a diskless workstation is booted, it gets its binary version of operating system from remote system. In such cases, the workstation broadcasts its MAC address. RARP server sees this MAC-address and finds its IP address from local configuration files and gives reply. However IP address can be coded into boot image itself. But since, several clients get boot image from some server, all may get the same IP address.
Reverse ARP demands the presence of RARP server on each network. Such Reverse ARP-requests can not be broadcasted onto other LANs. To avoid this problem, bootstrap protocol (BOOTP) was designed. RARP uses ethernet broadcast messages, whereas BOOTP uses UDP messages which can be forwarded over routers. A serious problem with BOOTP is it uses manual configuration of tasks mapping IP addresses to MAC address. When a new host is added, it can not use BOOTP until administrator assigns IP address to it and enters (IP, ethernet address) entry into BOOTP configuration tasks. To avoid it, it was extended to DHCP. This supports both manual and automatic addressing assignment.
DHCP will have DHCP-server, one for several LANs and a DHCP relay agent one per LAN. Since broadcast messages can not cross a LAN, DHCP relay agent receives such broadcasts and gets the information from DHCP server via a unicast connection. All needed by a DHCP relay agent is IP address of DHCP server. However IP addresses are assigned to hosts only on a ‘lease’ basis. Just before the lease period expires, host must ask the DHCP for renewal. If renewal request is denied, it has to try RARP again with another request.