Reverse address mapping allows a node name to be determined from an
 address (specifically from an NET).

 For directory services that support hierarchical naming, this is done by
 setting up directories and links within those directories, based on the
 NETs of the nodes in the network.  Each link then points to the node name
 associated with the indicated NET.

1  –  DECdns

 Soft link names are used to map NETs to their respective node names.

 The default value for the reverse address mapping directory in the
 DECdns directory service is:

   .DNA_BackTranslation

 If not specified using either of the above methods, the default Phase IV
 prefix is used.  This default can be displayed or changed using the SHOW
 or SET DEFAULT commands.

 The directory name can be changed for a single command using the
 REVERSE_MAPPING parameter, or for all commands using the SET DEFAULT
 command.

 There are two more levels of directory under this top level directory,
 followed by the actual soft link name.  The name structure is:

   .DNA_BackTranslation.<prefix>.<locarea>.<nodeid>

 Where:

   <prefix>      is the value of the binary NET up to the local area
                 field.

   <locarea>     is the value of the local area field in the binary NET.

   <nodeid>      is the value of the node id field in the binary NET.

 For example, for an NET of "39:840:00-01:AA-00-04-00-05-04:20", the
 fully specified soft link name would be:

   .DNA_BackTranslation.%X39840F.%X0001.%XAA0004000504

 The "F" in the second level directory is a padding value that is present
 in the binary form of the NSAP value, though it is not visible in the
 user representation.

2  –  Local

 The Local Naming Database does not make use of reverse address mapping
 for mapping addresses to names.  Instead, addresses are treated as keys
 within the indexed Local Database file.

3  –  Phase IV

 The Phase IV Database does not make use of reverse address mapping for
 mapping addresses to names.  Instead, addresses are treated as keys
 within the indexed Phase IV Database file.