Which of the following TCP/IP Internet protocols does a diskless machine use…

Concept: address binding in the TCP/IP suite runs in two opposite directions. One direction starts from a host's Internet address and asks which hardware (MAC) address owns it; the reverse direction starts from a hardware address and asks which Internet address has been assigned to it. A machine with no local storage knows only its burned-in hardware address at power-on, so its bootstrap problem is the reverse direction.

Application: a diskless machine keeps no configuration file on disk, so at boot it broadcasts a frame carrying its own hardware address on the local link, and a server holding the address table replies with the Internet address assigned to that hardware address. The lookup therefore runs in the hardware-address-to-Internet-address direction. Only the address-resolution family of the TCP/IP suite carries this kind of binding between a hardware address and an Internet address: ARP (RFC 826) defines the request/reply frame format for the Internet-address-to-hardware-address lookup, and RFC 903 defines the reverse-direction use of the very same frame, RARP, which is the protocol historically designed for diskless boot and later superseded by BOOTP and DHCP.

• ARP (RFC 826): the address-resolution exchange of the TCP/IP suite, whose broadcast request and unicast reply bind Internet addresses to hardware addresses on a local link.

• RIP: a distance-vector routing protocol in which routers exchange hop-count vectors so that each can build a routing table; it distributes reachability between routers.

• X.25: an ITU-T interface standard for packet-switched public data networks, where PLP over LAPB sets up and clears virtual circuits across the WAN.

• RAP: the Route Access Protocol of RFC 1476, an experimental inter-domain routing proposal that carries routing policy between administrative domains.

Cross-check and exam note: this item is Q.14 of UGC NET June 2012 Computer Science Paper-II (booklet J-87-12), where the four printed choices are exactly RAP, RIP, ARP and X.25, and UGC's official answer key for subject 87, Paper-II marks Q.14 as ARP. The original item carries a well-known flaw worth knowing: the protocol actually built for diskless address acquisition is RARP, and RARP does not appear among the printed choices, so the paper keys the address-resolution protocol ARP. For this examination answer ARP, while remembering that in practice a diskless host obtains its address through RARP, BOOTP or DHCP.