A Phase IV prefix is used to convert a Phase IV address to an NSAP.  It
 consists of the AFI, IDI, and preDSP fields of the NSAP.  The format is
 either of:

     DNA format -  <afi>:<idi>:<predsp>
     OSI format -  <afi><idi>+<predsp>

 Where:

     <afi>     - The AFI (Authority and Format Identifier)
     <idi>     - The IDI (Initial Domain Identifier)
     <predsp>  - The prefix portion of the DSP (Domain Specific Part)

 Each address field contains:

     AFI         2 decimal digits
     IDI         some number of decimal digits, determined by the AFI
     PreDSP      some number of hex digits, determined by the AFI

 A Phase IV prefix can be specified for an individual Phase IV address
 by adding it to the end of the address.  For example:

     1.5+39:840:

 A default Phase IV prefix can be specified for all Phase IV addresses
 using the preferences form.

 The local area and node id values are built from the Phase IV address,
 and the N-Sel value is generally "20" (for NSP), unless the Phase IV
 prefix is being applied to an address tower that specifies the use of
 TP4.