.\"	$OpenBSD: inet_addr.3,v 1.6 2022/09/11 06:38:10 jmc Exp $
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.\"     @(#)inet.3	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: September 11 2022 $
.Dt INET_ADDR 3
.Os
.Sh NAME
.Nm inet_aton ,
.Nm inet_addr ,
.Nm inet_network ,
.Nm inet_ntoa
.Nd Internet Protocol version 4 (IPv4) address manipulation routines
.Sh SYNOPSIS
.In arpa/inet.h
.Ft int
.Fn inet_aton "const char *cp" "struct in_addr *addr"
.Ft in_addr_t
.Fn inet_addr "const char *cp"
.Ft in_addr_t
.Fn inet_network "const char *cp"
.Ft char *
.Fn inet_ntoa "struct in_addr in"
.Sh DESCRIPTION
The functions presented here only support IPv4 addresses.
In order to support IPv6 addresses as well,
.Xr inet_ntop 3
and
.Xr inet_pton 3
should be used rather than the functions presented here.
Scoped IPv6 addresses are supported via
.Xr getaddrinfo 3
and
.Xr getnameinfo 3 .
.Pp
The routines
.Fn inet_aton ,
.Fn inet_addr ,
and
.Fn inet_network
interpret character strings representing
numbers expressed in the Internet standard
.Dq dot
notation.
.Pp
The
.Fn inet_aton
routine interprets the specified character string as an Internet address,
placing the address into the structure provided.
It returns 1 if the string was successfully interpreted,
or 0 if the string was invalid.
.Pp
The
.Fn inet_addr
and
.Fn inet_network
functions return numbers suitable for use
as Internet addresses and Internet network
numbers, respectively.
Both functions return the constant
.Dv INADDR_NONE
if the specified character string is malformed.
.Pp
The routine
.Fn inet_ntoa
takes an Internet address and returns an
ASCII string representing the address in dot notation.
.Pp
All Internet addresses are returned in network
order (bytes ordered from left to right).
All network numbers and local address parts are
returned as machine format integer values.
.Sh INTERNET ADDRESSES (IP VERSION 4)
Values specified using dot notation take one of the following forms:
.Bd -literal -offset indent
a.b.c.d
a.b.c
a.b
a
.Ed
.Pp
When four parts are specified, each is interpreted
as a byte of data and assigned, from left to right,
to the four bytes of an Internet address.
Note that when an Internet address is viewed as a 32-bit
integer quantity on a system that uses little-endian
byte order
(such as AMD64 or ARM processors)
the bytes referred to above appear as
.Dq Li d.c.b.a .
That is, little-endian bytes are ordered from right to left.
.Pp
When a three part address is specified, the last
part is interpreted as a 16-bit quantity and placed
in the rightmost two bytes of the network address.
This makes the three part address format convenient
for specifying Class B network addresses as
.Dq Li 128.net.host .
.Pp
When a two part address is supplied, the last part
is interpreted as a 24-bit quantity and placed in
the rightmost three bytes of the network address.
This makes the two part address format convenient
for specifying Class A network addresses as
.Dq Li net.host .
.Pp
When only one part is given, the value is stored
directly in the network address without any byte
rearrangement.
.Pp
All numbers supplied as
.Dq parts
in a dot notation
may be decimal, octal, or hexadecimal, as specified
in the C language (i.e., a leading 0x or 0X implies
hexadecimal; a leading 0 implies octal;
otherwise, the number is interpreted as decimal).
.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr htonl 3 ,
.Xr inet_lnaof 3 ,
.Xr inet_net_ntop 3 ,
.Xr inet_ntop 3 ,
.Xr hosts 5
.Sh STANDARDS
The
.Nm inet_addr
and
.Nm inet_aton
functions conform to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Nm inet_addr
and
.Nm inet_network
functions appeared in
.Bx 4.2 .
The
.Nm inet_aton
and
.Nm inet_ntoa
functions appeared in
.Bx 4.3 .
.Sh BUGS
The value
.Dv INADDR_NONE
(0xffffffff) is a valid broadcast address, but
.Fn inet_addr
cannot return that value without indicating failure.
Also,
.Fn inet_addr
should have been designed to return a
.Vt struct in_addr .
The newer
.Fn inet_aton
function does not share these problems, and almost all existing code
should be modified to use
.Fn inet_aton
instead.
.Pp
The problem of host byte ordering versus network byte ordering is
confusing.
.Pp
The string returned by
.Fn inet_ntoa
resides in a static memory area.