This specifies the AFI, IDI, and preDSP to use when constructing an
 NSAP from a Phase IV address.  This is used when a Phase IV address
 is specified in the ADDRESS parameter.

   PHASEIV_PREFIX addr-prefix

 For example:

     ADDRESS 1.5 PHASEIV_PREFIX 39:840:800AB738

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

     39:840:800AB738-0001:AA-00-04-00-05-04:20

 An alternate way of expressing this would be:

     ADDRESS 1.5+39:840:800AB738

 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.