This information must include at least the IP addresses of
              the host and of the boot server.  To support booting
              across a gateway, the address mask and a list of default
              gateways are also required.

         (2)  Load the host system code.

              During the loading phase, an appropriate file transfer
              protocol is used to copy the system code across the
              network from the boot server.

         A host with a disk may perform the first step, dynamic
         configuration.  This is important for microcomputers, whose
         floppy disks allow network configuration information to be
         mistakenly duplicated on more than one host.  Also,
         installation of new hosts is much simpler if they automatically
         obtain their configuration information from a central server,
         saving administrator time and decreasing the probability of
         mistakes.

      6.2.2  REQUIREMENTS

         6.2.2.1  Dynamic Configuration

            A number of protocol provisions have been made for dynamic
            configuration.

            o    ICMP Information Request/Reply messages




 



RFC1123            SUPPORT SERVICES -- INITIALIZATION       October 1989


                 This obsolete message pair was designed to allow a host
                 to find the number of the network it is on.
                 Unfortunately, it was useful only if the host already
                 knew the host number part of its IP address,
                 information that hosts requiring dynamic configuration
                 seldom had.

            o    Reverse Address Resolution Protocol (RARP) [BOOT:4]

                 RARP is a link-layer protocol for a broadcast medium
                 that allows a host to find its IP address given its
                 link layer address.  Unfortunately, RARP does not work
                 across IP gateways and therefore requires a RARP server
                 on every network.  In addition, RARP does not provide
                 any other configuration information.

            o    ICMP Address Mask Request/Reply messages

                 These ICMP messages allow a host to learn the address
                 mask for a particular network interface.

            o    BOOTP Protocol [BOOT:2]

                 This protocol allows a host to determine the IP
                 addresses of the local host and the boot server, the
                 name of an appropriate boot file, and optionally the
                 address mask and list of default gateways.  To locate a
                 BOOTP server, the host broadcasts a BOOTP request using
                 UDP.  Ad hoc gateway extensions have been used to
                 transmit the BOOTP broadcast through gateways, and in
                 the future the IP Multicasting facility will provide a
                 standard mechanism for this purpose.


            The suggested approach to dynamic configuration is to use
            the BOOTP protocol with the extensions defined in "BOOTP
            Vendor Information Extensions" RFC-1084 [BOOT:3].  RFC-1084
            defines some important general (not vendor-specific)
            extensions.  In particular, these extensions allow the
            address mask to be supplied in BOOTP; we RECOMMEND that the
            address mask be supplied in this manner.

            DISCUSSION:
                 Historically, subnetting was defined long after IP, and
                 so a separate mechanism (ICMP Address Mask messages)
                 was designed to supply the address mask to a host.
                 However, the IP address mask and the corresponding IP
                 address conceptually form a pair, and for operational




 



RFC1123            SUPPORT SERVICES -- INITIALIZATION       October 1989


                 simplicity they ought to be defined at the same time
                 and by the same mechanism, whether a configuration file