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.\"     @(#)inet.3	8.1 (Berkeley) 6/4/93
.\"
.Dd July 25, 2012
.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 ,
.Nm addr ,
.Nm ntoa ,
.Nm network
.Nd Internet address manipulation routines
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In 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 * restrict src" "char * restrict dst" "socklen_t size"
.Ft int
.Fn inet_pton "int af" "const char * restrict src" "void * restrict 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
.Qq dotted quad
notation.
.Pp
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
.Ft 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, or
0 if the address wasn't parsable 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 .
.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 is invalid.
.Pp
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
.Ft struct in_addr
or some other binary form, in network byte order) to presentation format
(suitable for external display purposes).
It returns 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
.Fa size
parameter is the size of the
.Fa dst
buffer.
For
.Dv AF_INET ,
this must have space for
.Dv INET_ADDRSTRLEN Pq 16
bytes; for
.Dv AF_INET6 ,
this must have space for
.Dv INET6_ADDRSTRLEN Pq 46
bytes.
.Pp
The routine
.Fn inet_ntoa
takes an Internet address and returns an
.Tn ASCII
string representing the address in
.Qq dotted quad
notation.
.Pp
The routine
.Fn inet_makeaddr
takes an Internet network number and a local network address (both in
host order) and constructs an Internet address from it.
Note that to convert only a single value to a
.Ft struct in_addr
form that value should be passed as the first parameter and
.Ql 0L
should be given for the second parameter.
.Pp
The routines
.Fn inet_netof
and
.Fn inet_lnaof
break apart Internet host addresses, returning the network number and
local network address part, respectively (both in host order).
.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 the
.Qq dotted quad
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 (e.g.
.Tn Intel i386, i486
and
.Tn 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 right-most 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 right most 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
.Qq dotted quad
notation
may be decimal, octal, or hexadecimal, as specified
in the C language (i.e., a leading 0x or 0X implies
hexadecimal; otherwise, 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,
the use of
.Xr getaddrinfo 3
and
.Xr getnameinfo 3
is 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 ``::'' indicates multiple groups of 16-bits of zeros.
The ``::'' can only appear once in an address.
The ``::'' 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 RETURN VALUES
The constant
.Dv INADDR_NONE
is returned by
.Fn inet_addr
and
.Fn inet_network
for malformed requests.
.Sh ERRORS
The
.Fn inet_ntop
and
.Fn inet_pton
functions may fail with
.Bl -tag -width Er
.It Bq Er EAFNOSUPPORT
The value of
.Fa af
was not
.Dv AF_INET
or
.Dv AF_INET6 .
.El
.Pp
The
.Fn inet_ntop
function may fail with
.Bl -tag -width Er
.It Bq Er ENOSPC
The
.Fa size
indicated for
.Fa dst
was too small to store the presentation form of the network address.
.El
.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
.Fn inet_ntop
and
.Fn inet_pton
functions conform to
.St -p1003.1-2001 .
Note that
.Fn inet_pton
does not accept 1-, 2-, or 3-part dotted addresses; all four parts
must be specified.
Additionally all four parts of a dotted address must be decimal.
This is a narrower input set than that accepted by
.Fn inet_aton .
.Sh HISTORY
The
.Fn inet_addr ,
.Fn inet_network ,
.Fn inet_makeaddr ,
.Fn inet_lnaof
and
.Fn inet_netof
functions appeared in
.Bx 4.2 .
They were changed to use
.Vt in_addr_t
in place of
.Vt unsigned long
in
.Nx 2.0 .
The
.Fn inet_aton
and
.Fn inet_ntoa
functions appeared in
.Bx 4.3 .
The
.Fn inet_pton
and
.Fn inet_ntop
functions appeared in BIND 4.9.4 and thence
.Nx 1.3 ;
they were also in
.St -xns5.2 .
.Sh BUGS
The value
.Dv INADDR_NONE
(0xffffffff) is a valid broadcast address, but
.Fn inet_addr
cannot return that value without indicating failure.
The newer
.Fn inet_aton
function does not share this problem.
.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.
.Pp
The function
.Fn inet_addr
should return a
.Vt struct in_addr .
.Pp
The function
.Fn inet_network
does not support byte rearrangement for one, two, and three
part addresses.