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.Dd $Mdocdate: June 12 2007 $
.Dt INET 3
.Os
.Sh NAME
.Nm inet_addr ,
.Nm inet_aton ,
.Nm inet_lnaof ,
.Nm inet_makeaddr ,
.Nm inet_netof ,
.Nm inet_network ,
.Nm inet_ntoa ,
.Nm inet_ntop ,
.Nm inet_pton
.Nd Internet address manipulation routines
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/socket.h>
.Fd #include <netinet/in.h>
.Fd #include <arpa/inet.h>
.Ft in_addr_t
.Fn inet_addr "const char *cp"
.Ft int
.Fn inet_aton "const char *cp" "struct in_addr *addr"
.Ft in_addr_t
.Fn inet_lnaof "struct in_addr in"
.Ft struct in_addr
.Fn inet_makeaddr "in_addr_t net" "in_addr_t lna"
.Ft in_addr_t
.Fn inet_netof "struct in_addr in"
.Ft in_addr_t
.Fn inet_network "const char *cp"
.Ft char *
.Fn inet_ntoa "struct in_addr in"
.Ft const char *
.Fn inet_ntop "int af" "const void *src" "char *dst" "size_t size"
.Ft int
.Fn inet_pton "int af" "const char *src" "void *dst"
.Sh DESCRIPTION
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.
The
.Fn inet_pton
function converts a presentation format address (that is, printable form
as held in a character string) to network format (usually a
.Li struct in_addr
or some other internal binary representation, in network byte order).
It returns 1 if the address was valid for the specified address family;
0 if the address wasn't parseable in the specified address family; or \-1
if some system error occurred (in which case
.Va errno
will have been set).
This function is presently valid for
.Dv AF_INET
and
.Dv AF_INET6 .
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.
The
.Fn inet_addr
and
.Fn inet_network
functions return numbers suitable for use
as Internet addresses and Internet network
numbers, respectively.
.Pp
The function
.Fn inet_ntop
converts an address from network format (usually a
.Li struct in_addr
or some other binary form, in network byte order) to presentation format
(suitable for external display purposes).
It returns
.Dv NULL
if a system
error occurs (in which case,
.Va errno
will have been set), or it returns a pointer to the destination string.
The routine
.Fn inet_ntoa
takes an Internet address and returns an
ASCII string representing the address in dot notation.
The routine
.Fn inet_makeaddr
takes an Internet network number and a local
network address and constructs an Internet address
from it.
The routines
.Fn inet_netof
and
.Fn inet_lnaof
break apart Internet host addresses, returning
the network number and local network address part,
respectively.
.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 the Intel 386, 486 and Pentium 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 INTERNET ADDRESSES (IP VERSION 6)
In order to support scoped IPv6 addresses,
.Xr getaddrinfo 3
and
.Xr getnameinfo 3
are recommended rather than the functions presented here.
.Pp
The presentation format of an IPv6 address is given in RFC 2373:
.Pp
There are three conventional forms for representing IPv6 addresses as
text strings:
.Bl -enum
.It
The preferred form is x:x:x:x:x:x:x:x, where the 'x's are the
hexadecimal values of the eight 16-bit pieces of the address.
Examples:
.Bd -literal -offset indent
FEDC:BA98:7654:3210:FEDC:BA98:7654:3210
1080:0:0:0:8:800:200C:417A
.Ed
.Pp
Note that it is not necessary to write the leading zeros in an
individual field, but there must be at least one numeral in
every field (except for the case described in 2.).
.It
Due to the method of allocating certain styles of IPv6
addresses, it will be common for addresses to contain long
strings of zero bits.
In order to make writing addresses
containing zero bits easier, a special syntax is available to
compress the zeros.
The use of
.Dq \&:\&:
indicates multiple groups
of 16 bits of zeros.
The
.Dq \&:\&:
can only appear once in an
address.
The
.Dq \&:\&:
can also be used to compress the leading and/or trailing zeros in an address.
.Pp
For example the following addresses:
.Bd -literal -offset indent
1080:0:0:0:8:800:200C:417A  a unicast address
FF01:0:0:0:0:0:0:43         a multicast address
0:0:0:0:0:0:0:1             the loopback address
0:0:0:0:0:0:0:0             the unspecified addresses
.Ed
.Pp
may be represented as:
.Bd -literal -offset indent
1080::8:800:200C:417A       a unicast address
FF01::43                    a multicast address
::1                         the loopback address
::                          the unspecified addresses
.Ed
.It
An alternative form that is sometimes more convenient when
dealing with a mixed environment of IPv4 and IPv6 nodes is
x:x:x:x:x:x:d.d.d.d, where the 'x's are the hexadecimal values
of the six high-order 16-bit pieces of the address, and the 'd's
are the decimal values of the four low-order 8-bit pieces of the
address (standard IPv4 representation).
Examples:
.Bd -literal -offset indent
0:0:0:0:0:0:13.1.68.3
0:0:0:0:0:FFFF:129.144.52.38
.Ed
.Pp
or in compressed form:
.Bd -literal -offset indent
::13.1.68.3
::FFFF:129.144.52.38
.Ed
.El
.Sh DIAGNOSTICS
The constant
.Dv INADDR_NONE
is returned by
.Fn inet_addr
and
.Fn inet_network
for malformed requests.
.Sh SEE ALSO
.Xr byteorder 3 ,
.Xr gethostbyname 3 ,
.Xr getnetent 3 ,
.Xr inet_net 3 ,
.Xr hosts 5 ,
.Xr networks 5
.Rs
.%R RFC 2373
.%D July 1998
.%T "IP Version 6 Addressing Architecture"
.Re
.Rs
.%R RFC 3493
.%D February 2003
.%T "Basic Socket Interface Extensions for IPv6"
.Re
.Sh STANDARDS
The
.Nm inet_ntop
and
.Nm inet_pton
functions conform to the IETF IPv6 BSD API and address formatting
specifications.
Note that
.Nm inet_pton
does not accept 1-, 2-, or 3-part dotted addresses; all four parts
must be specified.
This is a narrower input set than that accepted by
.Nm inet_aton .
.Sh HISTORY
The
.Nm inet_addr ,
.Nm inet_network ,
.Nm inet_makeaddr ,
.Nm inet_lnaof ,
and
.Nm inet_netof
functions appeared in
.Bx 4.2 .
The
.Nm inet_aton
and
.Nm inet_ntoa
functions appeared in
.Bx 4.3 .
The
.Nm inet_pton
and
.Nm inet_ntop
functions appeared in BIND 4.9.4.
.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
.Li 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.