1 Address_Towers
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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.
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2 Transport_Field
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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".
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2 Address_Field
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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.
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2 Examples
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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}
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