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

   .DNA_BackTranslation

 The default directory name can be changed using the preferences form.

 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 is:

   .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.