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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | ATTRIBUTES | STANDARDS | STANDARDS | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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inet(3) Library Functions Manual inet(3)
inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr,
inet_lnaof, inet_netof - Internet address manipulation routines
Standard C library (libc, -lc)
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int inet_aton(const char *cp, struct in_addr *inp);
in_addr_t inet_addr(const char *cp);
in_addr_t inet_network(const char *cp);
[[deprecated]] char *inet_ntoa(struct in_addr in);
[[deprecated]] struct in_addr inet_makeaddr(in_addr_t net,
in_addr_t host);
[[deprecated]] in_addr_t inet_lnaof(struct in_addr in);
[[deprecated]] in_addr_t inet_netof(struct in_addr in);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
inet_aton(), inet_ntoa():
Since glibc 2.19:
_DEFAULT_SOURCE
In glibc up to and including 2.19:
_BSD_SOURCE || _BSD_SOURCE
inet_aton() converts the Internet host address cp from the IPv4
numbers-and-dots notation into binary form (in network byte order)
and stores it in the structure that inp points to. inet_aton()
returns nonzero if the address is valid, zero if not. The address
supplied in cp can have one of the following forms:
a.b.c.d
Each of the four numeric parts specifies a byte of the
address; the bytes are assigned in left-to-right order to
produce the binary address.
a.b.c Parts a and b specify the first two bytes of the binary
address. Part c is interpreted as a 16-bit value that
defines the rightmost two bytes of the binary address.
This notation is suitable for specifying (outmoded) Class B
network addresses.
a.b Part a specifies the first byte of the binary address.
Part b is interpreted as a 24-bit value that defines the
rightmost three bytes of the binary address. This notation
is suitable for specifying (outmoded) Class A network
addresses.
a The value a is interpreted as a 32-bit value that is stored
directly into the binary address without any byte
rearrangement.
In all of the above forms, components of the dotted address can be
specified in decimal, octal (with a leading 0), or hexadecimal,
with a leading 0X). Addresses in any of these forms are
collectively termed IPV4 numbers-and-dots notation. The form that
uses exactly four decimal numbers is referred to as IPv4 dotted-
decimal notation (or sometimes: IPv4 dotted-quad notation).
inet_aton() returns 1 if the supplied string was successfully
interpreted, or 0 if the string is invalid (errno is not set on
error).
The inet_addr() function converts the Internet host address cp
from IPv4 numbers-and-dots notation into binary data in network
byte order. If the input is invalid, INADDR_NONE (usually -1) is
returned. Use of this function is problematic because -1 is a
valid address (255.255.255.255). Avoid its use in favor of
inet_aton(), inet_pton(3), or getaddrinfo(3), which provide a
cleaner way to indicate error return.
The inet_network() function converts cp, a string in IPv4 numbers-
and-dots notation, into a number in host byte order suitable for
use as an Internet network address. On success, the converted
address is returned. If the input is invalid, -1 is returned.
The inet_ntoa() function converts the Internet host address in,
given in network byte order, to a string in IPv4 dotted-decimal
notation. The string is returned in a statically allocated
buffer, which subsequent calls will overwrite.
The inet_lnaof() function returns the local network address part
of the Internet address in. The returned value is in host byte
order.
The inet_netof() function returns the network number part of the
Internet address in. The returned value is in host byte order.
The inet_makeaddr() function is the converse of inet_netof() and
inet_lnaof(). It returns an Internet host address in network byte
order, created by combining the network number.I net with the
local address host, both in host byte order.
The structure in_addr as used in inet_ntoa(), inet_makeaddr(),
inet_lnaof(), and inet_netof() is defined in <netinet/in.h> as:
typedef uint32_t in_addr_t;
struct in_addr {
in_addr_t s_addr;
};
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────┼───────────────┼────────────────┤
│ inet_aton(), inet_addr(), │ Thread safety │ MT-Safe locale │
│ inet_network(), inet_ntoa() │ │ │
├───────────────────────────────┼───────────────┼────────────────┤
│ inet_makeaddr(), │ Thread safety │ MT-Safe │
│ inet_lnaof(), inet_netof() │ │ │
└───────────────────────────────┴───────────────┴────────────────┘
inet_addr()
inet_ntoa()
POSIX.1-2008.
inet_aton()
None.
inet_addr()
inet_ntoa()
POSIX.1-2001, 4.3BSD.
inet_lnaof(), inet_netof(), and inet_makeaddr() are legacy
functions that assume they are dealing with classful network
addresses. Classful networking divides IPv4 network addresses
into host and network components at byte boundaries, as follows:
Class A
This address type is indicated by the value 0 in the most
significant bit of the (network byte ordered) address. The
network address is contained in the most significant byte,
and the host address occupies the remaining three bytes.
Class B
This address type is indicated by the binary value 10 in
the most significant two bits of the address. The network
address is contained in the two most significant bytes, and
the host address occupies the remaining two bytes.
Class C
This address type is indicated by the binary value 110 in
the most significant three bits of the address. The
network address is contained in the three most significant
bytes, and the host address occupies the remaining byte.
Classful network addresses are now obsolete, having been
superseded by Classless Inter-Domain Routing (CIDR), which divides
addresses into network and host components at arbitrary bit
(rather than byte) boundaries.
On x86 architectures, the host byte order is Least Significant
Byte first (little endian), whereas the network byte order, as
used on the Internet, is Most Significant Byte first (big endian).
An example of the use of inet_aton() and inet_ntoa() is shown
below. Here are some example runs:
$ ./a.out 226.000.000.037 # Last byte is in octal
226.0.0.31
$ ./a.out 0x7f.1 # First byte is in hex
127.0.0.1
Program source
#define _DEFAULT_SOURCE
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
struct in_addr addr;
if (argc != 2) {
fprintf(stderr, "%s <dotted-address>\n", argv[0]);
exit(EXIT_FAILURE);
}
if (inet_aton(argv[1], &addr) == 0) {
fprintf(stderr, "Invalid address\n");
exit(EXIT_FAILURE);
}
printf("%s\n", inet_ntoa(addr));
exit(EXIT_SUCCESS);
}
byteorder(3), getaddrinfo(3), gethostbyname(3), getnameinfo(3),
getnetent(3), inet_net_pton(3), inet_ntop(3), inet_pton(3),
hosts(5), networks(5)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 inet(3)
Pages that refer to this page: getaddrinfo(3), getaddrinfo_a(3), gethostbyname(3), inet_net_pton(3), inet_ntop(3), inet_pton(3), networks(5), ip(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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