Address towers are used in the REGISTER, MODIFY, ADD, and REMOVE
 commands, by specifying the TOWERS parameter.  The format for a TOWERS
 parameter is:

     TOWERS {tower, tower, ...}

 Each address tower in the set has the format:

     transport/address

 Fields can be omitted from left to right, and assume a default value.

1  –  Transport Field

 This field indicates the Transport type.  This can be either:

     TP4     - OSI Transport Class 4
     NSP     - DECnet Phase IV Network Services Protocol

 If TP4 is specified, it can be followed by an optional T-Sel value as a
 series of hexadecimal digits after an equals sign.  If an explicit T-Sel
 value is not specified, the DECnet default of "TP4=DEC0" is assumed.
 For example:

     TP4=AB12  - The T-Sel value is set to "AB12"
     TP4=AB1   - The T-Sel value is "AB10" (number of digits must be even)
     TP4       - The T-Sel value is assumed to be "DEC0"

 If a transport type is not specified, the default depends on the address.

     NSP is used if the address is specified using Phase IV format, or
     is an NSAP with an N-Sel value of "20".

     TP4 is used if the address is an NSAP with an N-Sel value other
     than "20".

2  –  Address Field

 This field indicates the node address.  This can be specified using
 either Phase IV format or NSAP format.

   Phase IV format:  <area>.<nodeid>
                     <area>.<nodeid>+<prefix>

   DNA NSAP format:  <afi>:<idi>:<predsp>-<locarea>:<nodeid>:<nsel>
   OSI NSAP format:  <afi><idi>+<predsp><locarea><nodeid><nsel>

 See PHASE_IV_ADDRESSES and NSAP_ADDRESSES for more details.

3  –  Examples

 Assuming that the Phase IV address prefix value is "49::", some examples
 of address towers are:

   Abbreviation                      Fully Specified
   ------------                      ---------------
   1.5                               NSP/49::01:AA0004000504:20
   1.5                               NSP/49::01:AA0004000504:20
   39:840:01:AA0004000504:20         NSP/39:840:01:AA0004000504:20
   39:840:01:AA0004000504:21         TP4=DEC0/39:840:01:AA0004000504:21
   TP4/39:840:01:AA0004000504:21     TP4=DEC0/39:840:01:AA0004000504:21

 Examples of address towers using normal default values for both a DECnet
 Phase IV node and a DECnet-Plus node might be:

   TOWERS={1.5}
   TOWERS={2.54, 39:840:01:AA0004000504:20, 39:840:01:AA0004000504:21}