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.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\"     libc.info (from glibc distribution)
.\" Modified Sat Jul 24 19:12:00 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Sun Sep  3 20:29:36 1995 by Jim Van Zandt <jrv@vanzandt.mv.com>
.\" Changed network into host byte order (for inet_network),
.\"     Andreas Jaeger <aj@arthur.rhein-neckar.de>, 980130.
.\" 2008-06-19, mtk
.\"     Describe the various address forms supported by inet_aton().
.\"     Clarify discussion of inet_lnaof(), inet_netof(), and inet_makeaddr().
.\"     Add discussion of Classful Addressing, noting that it is obsolete.
.\"     Added an EXAMPLE program.
.\"
.TH INET 3  2008-06-19 "GNU" "Linux Programmer's Manual"
.SH NAME
inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr, inet_lnaof,
inet_netof \- Internet address manipulation routines
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.B #include <netinet/in.h>
.B #include <arpa/inet.h>
.sp
.BI "int inet_aton(const char *" cp ", struct in_addr *" inp );
.sp
.BI "in_addr_t inet_addr(const char *" cp );
.sp
.BI "in_addr_t inet_network(const char *" cp );
.sp
.BI "char *inet_ntoa(struct in_addr " in );
.sp
.BI "struct in_addr inet_makeaddr(int " net ", int " host );
.sp
.BI "in_addr_t inet_lnaof(struct in_addr " in );
.sp
.BI "in_addr_t inet_netof(struct in_addr " in );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR inet_aton (),
.BR inet_ntoa ():
_BSD_SOURCE || _SVID_SOURCE
.SH DESCRIPTION
.BR inet_aton ()
converts the Internet host address \fIcp\fP from the
IPv4 numbers-and-dots notation into binary form (in network byte order)
and stores it in the structure that \fIinp\fP points to.
.BR inet_aton ()
returns non-zero if the address is valid, zero if not.
The address supplied in
.I cp
can have one of the following forms:
.TP 10
.I 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.
.TP
.I a.b.c
Parts
.I a
and
.I b
specify the first two bytes of the binary address.
Part
.I 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.
.TP
.I a.b
Part
.I a
specifies the first byte of the binary address.
Part
.I 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 C
network addresses.
.TP
.I a
The value
.I a
is interpreted as a 32-bit value that is stored directly
into the binary address without any byte rearrangement.
.PP
In all of the above forms,
components of the dotted address can be specified in decimal,
octal (with a leading
.IR 0 ),
or hexadecimal, with a leading
.IR 0X ).
Addresses in any of these forms are collectively termed
.IR "IPV4 numbers-and-dots notation" .
The form that uses exactly four decimal numbers is referred to as
.IR "IPv4 dotted-decimal notation"
(or sometimes:
.IR "IPv4 dotted-quad notation" ).
.PP
The
.BR inet_addr ()
function converts the Internet host address
\fIcp\fP from IPv4 numbers-and-dots notation into binary data in network
byte order.
If the input is invalid,
.B 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
.BR inet_aton (),
.BR inet_pton (3),
or
.BR getaddrinfo (3)
which provide a cleaner way to indicate error return.
.PP
The
.BR inet_network ()
function converts
.IR 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.
.PP
The
.BR inet_ntoa ()
function converts the Internet host address
\fIin\fP, 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.
.PP
The
.BR inet_lnaof ()
function returns the local network address part
of the Internet address \fIin\fP.
The returned value is in host byte order.
.PP
The
.BR inet_netof ()
function returns the network number part of
the Internet address \fIin\fP.
The returned value is in host byte order.
.PP
The
.BR inet_makeaddr ()
function is the converse of
.BR inet_netof ()
and
.BR inet_lnaof ().
It returns an Internet host address in network byte order,
created by combining the network number \fInet\fP
with the local address \fIhost\fP, both in
host byte order.
.PP
The structure \fIin_addr\fP as used in
.BR inet_ntoa (),
.BR inet_makeaddr (),
.BR inet_lnaof ()
and
.BR inet_netof ()
is defined in
.I <netinet/in.h>
as:
.sp
.in +4n
.nf
typedef uint32_t in_addr_t;

struct in_addr {
    in_addr_t s_addr;
};
.fi
.in
.SH "CONFORMING TO"
4.3BSD.
.BR inet_addr ()
and
.BR inet_ntoa ()
are specified in POSIX.1-2001.
.BR inet_aton ()
is not specified in POSIX.1-2001, but is available on most systems.
.SH NOTES
On the i386 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).

.BR inet_lnaof (),
.BR inet_netof (),
and
.BR inet_makeaddr ()
are legacy functions that assume they are dealing with
.IR "classful network addresses" .
Classful networking divides IPv4 network addresses into host and network
components at byte boundaries, as follows:
.TP 10
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.
.TP
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.
.TP
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.
.PP
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.
.SH EXAMPLE
An example of the use of
.BR inet_aton ()
and
.BR inet_ntoa ()
is shown below.
Here are some example runs:
.in +4n
.nf

.RB "$" " ./a.out 226.000.000.037" "      # Last byte is in octal"
226.0.0.31
.RB "$" " ./a.out 0x7f.1         " "      # First byte is in hex"
127.0.0.1
.fi
.in
.SS Program source
\&
.nf
#define _BSD_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) {
        perror("inet_aton");
        exit(EXIT_FAILURE);
    }

    printf("%s\\n", inet_ntoa(addr));
    exit(EXIT_SUCCESS);
}
.fi
.SH "SEE ALSO"
.BR byteorder (3),
.BR getaddrinfo (3),
.BR gethostbyname (3),
.BR getnameinfo (3),
.BR getnetent (3),
.BR inet_ntop (3),
.BR inet_pton (3),
.BR hosts (5),
.BR networks (5)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
