This specifies the default 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, and an explicit Phase IV prefix is not
 specified.

     PHASEIV_PREFIX addr-prefix

 For example:

     SET DEFAULT PHASEIV_PREFIX 39:840:800AB738

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