This specifies the AFI, IDI, and preDSP to use when constructing an
 NET from a Phase IV address.  This is used only when a Phase IV address
 is specified for the node id.

   PHASEIV_PREFIX addr-prefix

 An alternate form is to add the Phase IV prefix to the Phase IV address
 specification.

 For example:

     NODE 1.5 PHASEIV_PREFIX 39:840:800AB738
     NODE 1.5+39:840:800AB738

 would both result in the following NET being used to represent the
 DECnet Phase IV address:

     39:840:800AB738-0001:AA-00-04-00-05-04:00

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

 See the PHASE_IV_PREFIXES topic for details about Phase IV prefix values.