p1
    OpenVMS usage:subfunction_code
    type:         longword (unsigned)
    access:       read only
    mechanism:    by descriptor-fixed-length descriptor
    A longword identifying the network ACP operation to perform.
    The p1 argument is the address of a descriptor pointing to this
    longword.
    To specify the network ACP operation to perform, select a
    subfunction code from the first table below and a call code from
    the second table.
    The following table defines subfunction codes for network ACP
    operations.
 .
    Subfunction Code            Description
    INETACP_FUNC$C_             Get the host name of the specified IP
    GETHOSTBYADDR               address from the hosts database.
    INETACP_FUNC$C_             Get the IP address of the specified
    GETHOSTBYNAME               host from the hosts database.
    INETACP_FUNC$C_             Get the network name of the specified
    GETNETBYADDR                IP address from the network database.
    INETACP_FUNC$C_             Get the IP address of the specified
    GETNETBYNAME                network from the network database.
    The following table defines call codes for network ACP
    operations.
    Call Code              Description
    INETACP$C_ALIASES      Returns the list of alias names associated
                           with the specified host or network from
                           the internet hosts or network database.
    INETACP$C_TRANS        Returns the IP address associated with
                           the specified host or network as a 32-bit
                           value in network byte order.
    INETACPC$C_HOSTENT_    Returns full host information in a
    OFFSET                 modified hostent structure. In the
                           modified structure, pointers are replaced
                           with offsets from the beginning of the
                           structure.
    INETACP$C_NETENT_      Returns full network information in
    OFFSET                 a modified netent structure. In the
                           modified structure, pointers are replaced
                           with offsets from the beginning of the
                           structure.
    IO$_ACPCONTROL searches the local hosts database for the host's
    name. If a matching host name is not found in the local hosts
    database, IO$_ACPCONTROL then searches the BIND database if the
    BIND resolver is enabled.
 p2
    OpenVMS usage:char_string
    type:         character-coded text string
    access:       read only
    mechanism:    by descriptor-fixed-length string descriptor
    Input string for the network ACP operation containing one of
    the following: host IP address, host name, network IP address,
    or network name. The p2 argument is the address of a string
    descriptor pointing to the input string. The input string must
    be in an area of memory that is capable of being read and written
    to.
    All IP addresses are specified in dotted-decimal notation.
 p3
    OpenVMS usage:word_unsigned
    type:         word (unsigned)
    access:       write only
    mechanism:    by reference
    Length in bytes of the output buffer returned by IO$_ACPCONTROL.
    The p3 argument is the address of a word in which the length of
    the output buffer is written.
 p4
    OpenVMS usage:buffer
    type:         vector byte (unsigned)
    access:       write only
    mechanism:    by descriptor-fixed-length descriptor
    Buffer into which IO$_ACPCONTROL writes its output data. The p4
    argument is the address of a descriptor pointing to the output
    buffer.
    The format of the data returned in the output buffer is dictated
    by the call code specified by the p1 argument.
    o  Strings returned by IO$_ACPCONTROL with a call code of
       INETACP$C_ALIASES consist of one of the following: host IP
       address, host name, network IP address, or network name. All
       IP addresses are formatted using dotted-decimal notation.
       Alias names are separated by a null character (0). The length
       of the returned string includes all null characters that
       separate alias names.
    o  IP addresses returned by IO$_ACPCONTROL with a call code of
       INETACP$C_TRANS are 32-bit values in network byte order.
    o  All hostent and netent structures returned by IO$_
       ACPCONTROL with a call code of INETACP$C_HOSTENT_OFFSET or
       INETACP$C_NETENT_OFFSET are modified; pointers are replaced
       with offsets from the beginning of the structure.