inet_net_pton, inet_net_ntop − Internet network number conversion


#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:


Before glibc 2.20:



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.

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 typically 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.

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 dotted-decimal number representing the network address, followed by a slash, and the size of the network number in bits.


On success, inet_net_pton() returns the number of bits in the network number. On error, it returns −1, and errno is set to indicate the cause of the error.

On success, inet_net_ntop() returns pres. On error, it returns NULL, and errno is set to indicate the cause of the error.



af specified a value other than AF_INET.


The size of the output buffer was insufficient.


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


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


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 hexadecimal 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:


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 network 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 greater than or equal to 128, then bits is 16.


Otherwise, bits is 8.

If the resulting bits value from the above steps is greater than or equal to 8, but the number of octets specified in the network number exceed bits/8, then bits is set to 8 times the number of octets actually specified.


The program below demonstrates the use of inet_net_pton() and inet_net_ntop(). It uses inet_net_pton() to convert the presentation 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
inet_net_pton() returned: 32
inet_net_ntop() yielded:
Raw address: c1a80180

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

$ ./a.out
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>

#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)

main(int argc, char *argv[])
char buf[100];
struct in_addr addr;
int bits;

if (argc < 2) {
"Usage: %s presentation−form [addr−init−value]\n",

/* 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)

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)

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));



inet(3), networks(5)


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