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 an address tower.

   PHASEIV_PREFIX addr-prefix

 For example:

     TOWERS {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:

     TOWERS {1.5+39:840:800AB738}

 The advantage of using the PHASEIV_PREFIX method is that the address
 prefix is applied to all Phase IV addresses without being specified
 multiple times.

 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.