A host may use a local host table as a backup or supplement to the DNS. This raises the question of which takes precedence, the DNS or the host table; the most flexible approach would make this a configuration option.
Typically, the contents of such a supplementary host table will be determined locally by the site. However, a publically-available table of Internet hosts is maintained by the DDN Network Information Center (DDN NIC), with a format documented in [DNS:4]. This table can be retrieved from the DDN NIC using a protocol described in [DNS:5]. It must be noted that this table contains only a small fraction of all Internet hosts. Hosts using this protocol to retrieve the DDN NIC host table should use the VERSION command to check if the table has changed before requesting the entire table with the ALL command. The VERSION identifier should be treated as an arbitrary string and tested only for equality; no numerical sequence may be assumed.
The DDN NIC host table includes administrative information that is not needed for host operation and is therefore not currently included in the DNS database; examples include network and gateway entries. However, much of this additional information will be added to the DNS in the future. Conversely, the DNS provides essential services (in particular, MX records) that are not available from the DDN NIC host table.