inet_net_ntop

       #include <arpa/inet.h>

       int inet_net_pton(int af, const char *pres,
                         void *netp, size_t nsize);

       char *inet_net_ntop(int af, const void *netp, int bits,
                           char *pres, size_t psize);

       Link with -lresolv.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       inet_net_pton(), inet_net_ntop():
           Since glibc 2.20:
               _DEFAULT_SOURCE
           Before glibc 2.20:
               _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
       These  functions  convert  network  numbers between presentation (i.e.,
       printable) format and network (i.e., binary) format.

       For both functions, af specifies the address family for the conversion;
       the only supported value is AF_INET.

   inet_net_pton()
       The  inet_net_pton()  function  converts pres, a null-terminated string
       containing an Internet network number in presentation format to network
       format.   The result of the conversion, which is in network byte order,
       is placed in the buffer pointed to by net.  (The  netp  argument  typi-
       cally  points  to  an in_addr structure.)  The nsize argument specifies
       the number of bytes available in netp.

       On success, inet_net_pton() returns the number of bits in  the  network
       number  field  of  the  result placed in netp.  For a discussion of the
       input presentation format and the return value, see NOTES.

       Note: the buffer pointed to by netp should be zeroed out before calling
       inet_net_pton(),  since  the  call  writes  only  as  many bytes as are
       required for the network number (or  as  are  explicitly  specified  by
       pres), which may be less than the number of bytes in a complete network
       address.

   inet_net_ntop()
       The inet_net_ntop() function converts the network number in the  buffer
       pointed  to  by  netp to presentation format; *netp is interpreted as a
       value in network byte order.  The bits argument specifies the number of
       bits in the network number in *netp.

       The  null-terminated presentation-format string is placed in the buffer
       pointed to by pres.  The psize argument specifies the number  of  bytes
       available  in  pres.  The presentation string is in CIDR format: a dot-
       ted-decimal number representing the  network  address,  followed  by  a
       slash, and the size of the network number in bits.

       EMSGSIZE
              The size of the output buffer was insufficient.

       ENOENT (inet_net_pton()) pres was not in correct presentation format.

CONFORMING TO
       The inet_net_pton() and inet_net_ntop() functions are nonstandard,  but
       widely available.

NOTES
   Input presentation format for inet_net_pton()
       The network number may be specified either as a hexadecimal value or in
       dotted-decimal notation.

       Hexadecimal values are indicated by an initial "0x" or "0X".  The hexa-
       decimal digits populate the nibbles (half octets) of the network number
       from left to right in network byte order.

       In dotted-decimal notation, up to four octets are specified, as decimal
       numbers  separated  by  dots.   Thus,  any  of  the following forms are
       accepted:

           a.b.c.d
           a.b.c
           a.b
           a

       Each part is a number in the range 0 to 255 that populates one byte  of
       the resulting network number, going from left to right, in network-byte
       (big endian) order.  Where a part is omitted, the resulting byte in the
       network number is zero.

       For either hexadecimal or dotted-decimal format, the network number can
       optionally be followed by a slash and a number in the range  0  to  32,
       which specifies the size of the network number in bits.

   Return value of inet_net_pton()
       The  return  value of inet_net_pton() is the number of bits in the net-
       work number field.  If the input presentation string terminates with  a
       slash  and  an  explicit  size value, then that size becomes the return
       value of  inet_net_pton().   Otherwise,  the  return  value,  bits,  is
       inferred as follows:

       *  If  the  most significant byte of the network number is greater than
          or equal to 240, then bits is 32.

       *  Otherwise, if the most significant byte of  the  network  number  is
          greater than or equal to 224, then bits is 4.

       *  Otherwise,  if  the  most  significant byte of the network number is
          greater than or equal to 192, then bits is 24.

       *  Otherwise, if the most significant byte of  the  network  number  is
       format network address provided in its first command-line  argument  to
       binary  form,  displays the return value from inet_net_pton().  It then
       uses inet_net_ntop() to convert the binary form  back  to  presentation
       format, and displays the resulting string.

       In order to demonstrate that inet_net_pton() may not write to all bytes
       of its netp argument, the program allows an  optional  second  command-
       line   argument,   a  number  used  to  initialize  the  buffer  before
       inet_net_pton() is called.  As its final line of  output,  the  program
       displays  all  of  the  bytes of the buffer returned by inet_net_pton()
       allowing the  user  to  see  which  bytes  have  not  been  touched  by
       inet_net_pton().

       An  example run, showing that inet_net_pton() infers the number of bits
       in the network number:

           $ ./a.out 193.168
           inet_net_pton() returned: 24
           inet_net_ntop() yielded:  193.168.0/24
           Raw address:              c1a80000

       Demonstrate that inet_net_pton() does not zero out unused bytes in  its
       result buffer:

           $ ./a.out 193.168 0xffffffff
           inet_net_pton() returned: 24
           inet_net_ntop() yielded:  193.168.0/24
           Raw address:              c1a800ff

       Demonstrate  that  inet_net_pton()  will widen the inferred size of the
       network number, if the supplied number of  bytes  in  the  presentation
       string exceeds the inferred value:

           $ ./a.out 193.168.1.128
           inet_net_pton() returned: 32
           inet_net_ntop() yielded:  193.168.1.128/32
           Raw address:              c1a80180

       Explicitly  specifying  the  size  of  the network number overrides any
       inference about its size (but any extra bytes that are explicitly spec-
       ified  will  still  be  used by inet_net_pton(): to populate the result
       buffer):

           $ ./a.out 193.168.1.128/24
           inet_net_pton() returned: 24
           inet_net_ntop() yielded:  193.168.1/24
           Raw address:              c1a80180

   Program source
       /* Link with "-lresolv" */

       #include <arpa/inet.h>
       #include <stdio.h>
       #include <stdlib.h>
               fprintf(stderr,
                       "Usage: %s presentation-form [addr-init-value]\n",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           /* If argv[2] is supplied (a numeric value), use it to initialize
              the output buffer given to inet_net_pton(), so that we can see
              that inet_net_pton() initializes only those bytes needed for
              the network number. If argv[2] is not supplied, then initialize
              the buffer to zero (as is recommended practice). */

           addr.s_addr = (argc > 2) ? strtod(argv[2], NULL) : 0;

           /* Convert presentation network number in argv[1] to binary */

           bits = inet_net_pton(AF_INET, argv[1], &addr, sizeof(addr));
           if (bits == -1)
               errExit("inet_net_ntop");

           printf("inet_net_pton() returned: %d\n", bits);

           /* Convert binary format back to presentation, using 'bits'
              returned by inet_net_pton() */

           if (inet_net_ntop(AF_INET, &addr, bits, buf, sizeof(buf)) == NULL)
               errExit("inet_net_ntop");

           printf("inet_net_ntop() yielded:  %s\n", buf);

           /* Display 'addr' in raw form (in network byte order), so we can
              see bytes not displayed by inet_net_ntop(); some of those bytes
              may not have been touched by inet_net_ntop(), and so will still
              have any initial value that was specified in argv[2]. */

           printf("Raw address:              %x\n", htonl(addr.s_addr));

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       inet(3), networks(5)

COLOPHON
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       latest    version    of    this    page,    can     be     found     at
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Linux                             2014-05-28                  INET_NET_PTON(3)
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