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.\" Copyright 1989 AT&T
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.Dd July 22, 2018
.Dt INET 3C
.Os
.Sh NAME
.Nm inet ,
.Nm inet6 ,
.Nm inet_ntop ,
.Nm inet_pton ,
.Nm inet_aton ,
.Nm inet_addr ,
.Nm inet_network ,
.Nm inet_makeaddr ,
.Nm inet_lnaof ,
.Nm inet_netof ,
.Nm inet_ntoa
.Nd Internet address manipulation
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netinet/in.h
.In arpa/inet.h
.Ft const char *
.Fo inet_ntop
.Fa "int af"
.Fa "const void *addr"
.Fa "char *cp"
.Fa "size_t size"
.Fc
.Ft int
.Fo inet_pton
.Fa "int af"
.Fa "const char *cp"
.Fa "void *addr"
.Fc
.Ft int
.Fo inet_aton
.Fa "const char *cp"
.Fa "struct in_addr *addr"
.Fc
.Ft in_addr_t
.Fo inet_addr
.Fa "const char *cp"
.Fc
.Ft in_addr_t
.Fo inet_network
.Fa "const char *cp"
.Fc
.Ft struct in_addr
.Fo inet_makeaddr
.Fa "const int net"
.Fa "const int lna"
.Fc
.Ft in_addr_t
.Fo inet_lnaof
.Fa "const struct in_addr in"
.Fc
.Ft in_addr_t
.Fo inet_netof
.Fa "const struct in_addr in"
.Fc
.Ft char *
.Fo inet_ntoa
.Fa "const struct in_addr in"
.Fc
.Sh DESCRIPTION
The
.Fn inet_ntop
and
.Fn inet_pton
functions can manipulate both IPv4 and IPv6 addresses.
The
.Fn inet_aton ,
.Fn inet_addr ,
.Fn inet_network ,
.Fn inet_makeaddr ,
.Fn inet_lnaof ,
.Fn inet_netof ,
and
.Fn inet_ntoa
functions can only manipulate IPv4 addresses.
.Pp
The
.Fn inet_ntop
function converts a numeric address into a string suitable for presentation.
The
.Fa af
argument specifies the family of the address which can be
.Dv AF_INET
or
.Dv AF_INET6 .
The
.Fa addr
argument points to a buffer that holds an IPv4 address if the
.Fa af
argument is
.Dv AF_INET .
The
.Fa addr
argument points to a buffer that holds an IPv6 address if the
.Fa af
argument is
.Dv AF_INET6 .
The address must be in network byte order.
The
.Fa cp
argument points to a buffer where the function stores the resulting string.
The application must specify a non-NULL
.Fa cp
argument.
The
.Fa size
argument specifies the size of this buffer.
For IPv6 addresses, the buffer must be at least 46-octets.
For IPv4 addresses, the buffer must be at least 16-octets.
To allow applications to easily declare buffers of the proper size to store IPv4
and IPv6 addresses in string form, the following two constants are defined in
.In netinet/in.h :
.Bd -literal
#define INET_ADDRSTRLEN    16
#define INET6_ADDRSTRLEN   46
.Ed
.Pp
The
.Fn inet_pton
function converts the standard text presentation form of a function to the
numeric binary form.
The
.Fa af
argument specifies the family of the address.
Currently, the
.Dv AF_INET
and
.Dv AF_INET6
address families are supported.
The
.Fa cp
argument points to the string being passed in.
The
.Fa addr
argument points to a buffer where the function stores the numeric address.
The calling application must ensure that the buffer referred to by
.Fa addr
is large enough to hold the numeric address, at least 4 bytes for
.Dv AF_INET
or 16 bytes for
.Dv AF_INET6 .
.Pp
The
.Fn inet_aton ,
.Fn inet_addr ,
and
.Fn inet_network
functions interpret character strings that represent numbers expressed in the
IPv4 standard
.Ql \&.
notation, returning numbers suitable for use as IPv4 addresses and IPv4 network
numbers, respectively.
The
.Fn inet_makeaddr
function uses an IPv4 network number and a local network address to construct
an IPv4 address.
The
.Fn inet_netof
and
.Fn inet_lnaof
functions break apart IPv4 host addresses, then return the network number and
local network address, respectively.
.Pp
The
.Fn inet_ntoa
function returns a pointer to a string in the base 256 notation
.Ql d.d.d.d .
See the following section on IPv4 addresses.
.Pp
Internet addresses are returned in network order, bytes ordered from left to
right.
Network numbers and local address parts are returned as machine format integer
values.
.Ss IPv6 Addresses
There are three conventional forms for representing IPv6 addresses as strings:
.Bl -enum
.It
The preferred form is
.Ql x:x:x:x:x:x:x:x ,
where the
.Li x No 's
are the
hexadecimal values of the eight 16-bit pieces of the address.
For example,
.Ql 1080:0:0:0:8:800:200C:417A .
.Pp
It is not necessary to write the leading zeros in an individual field.
There must be at least one numeral in every field, except when the special
syntax described in the following is used.
.It
It is common for addresses to contain long strings of zero bits in some
methods used to allocate certain IPv6 address styles.
A special syntax is available to compress the zeros.
The use of
.Ql ::
indicates multiple groups of 16 bits of zeros.
The
.Ql ::
may only appear once in an address.
The
.Ql ::
can also be used to compress the leading and trailing zeros in an address.
For example,
.Ql 1080::8:800:200C:417A .
.It
The alternative form
.Ql x:x:x:x:x:x:d.d.d.d
is sometimes more convenient when dealing with a mixed environment of IPv4 and
IPv6 nodes.
The
.Li x No 's
in this form represent the hexadecimal values of the six high-order 16-bit
pieces of the address.
The
.Li d No 's
represent the decimal values of the four low-order 8-bit pieces of the standard
IPv4 address.
For example:
.Bd -literal
::FFFF:129.144.52.38 .
::129.144.52.38
.Ed
.Pp
The
.Ql ::FFFF:d.d.d.d
and
.Ql ::d.d.d.d
pieces are the general forms of an IPv4-mapped IPv6 address and an
IPv4-compatible IPv6 address.
.Pp
The IPv4 portion must be in the
.Ql d.d.d.d
form.
The following forms are invalid:
.Bd -literal
::FFFF:d.d.d
::FFFF:d.d
::d.d.d
::d.d
.Ed
.Pp
The
.Ql ::FFFF:d
form is a valid but unconventional representation of the IPv4-compatible IPv6
address
.Ql ::255.255.0.d .
.Pp
The
.Ql ::d
form corresponds to the general IPv6 address
.Ql 0:0:0:0:0:0:0:d .
.El
.Ss IPv4 Addresses
Values specified using
.Ql \&.
notation take one of the following forms:
.Bd -literal
d.d.d.d
d.d.d
d.d
d
.Ed
.Pp
When four parts are specified, each part is interpreted as a byte of data and
assigned from left to right to the four bytes of an IPv4 address.
.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.
The three part address format is convenient for specifying Class B network
addresses such as
.Ql 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.
The two part address format is convenient for specifying Class A network
addresses such as
.Ql net.host .
.Pp
When only one part is given, the value is stored directly in the network
address without any byte rearrangement.
.Pp
With the exception of
.Fn inet_pton ,
numbers supplied as parts in
.Ql \&.
notation may be decimal, octal, or hexadecimal, as specified in C language.
For example, a leading
.Ql 0x
or
.Ql 0X
implies hexadecimal.
A leading
.Ql 0
implies octal.
Otherwise, the number is interpreted as decimal.
.Pp
For IPv4 addresses,
.Fn inet_pton
accepts only a string in standard IPv4 dot notation
.Ql d.d.d.d .
.Pp
Each number has one to three digits with a decimal value between 0 and 255.
.Pp
The
.Fn inet_addr
function has been obsoleted by
.Fn inet_aton .
.Sh RETURN VALUES
The
.Fn inet_aton
function returns nonzero if the address is valid,
.Li 0
if the address is invalid.
.Pp
The
.Fn inet_ntop
function returns a pointer to the buffer that contains a string if the
conversion succeeds.
Otherwise,
.Dv NULL
is returned.
Upon failure,
.Va errno
is set to
.Er EAFNOSUPPORT
if the
.Fa af
argument is invalid or
.Er ENOSPC
if the size of the result buffer is inadequate.
.Pp
The
.Fn inet_pton
function returns
.Li 1
if the conversion succeeds,
.Li 0
if the input is not a valid IPv4 dotted-decimal string or a valid IPv6
address string.
The function returns
.Li -1
with
.Va errno
set to
.Er EAFNOSUPPORT
if the
.Fa af
argument is unknown.
.Pp
The value
.Dv INADDR_NONE ,
which is equivalent to
.Li (in_addr_t)(-1) ,
is returned by
.Fn inet_addr
and
.Fn inet_network
for malformed requests.
.Pp
The functions
.Fn inet_netof
and
.Fn inet_lnaof
break apart IPv4 host addresses, returning the network number and local network
address part, respectively.
.Pp
The function
.Fn inet_ntoa
returns a pointer to a string in the base 256 notation
.Ql d.d.d.d ,
described in the section on IPv4 addresses.
.Sh MT-LEVEL
.Sy Safe
.Sh INTERFACE STABILITY
The
.Fn inet_ntop ,
.Fn inet_pton ,
.Fn inet_aton ,
.Fn inet_addr ,
and
.Fn inet_network
functions are
.Sy Committed .
The
.Fn inet_lnaof ,
.Fn inet_makeaddr ,
.Fn inet_netof ,
and
.Fn inet_network
functions are
.Sy Obsolete Committed .
.Sh SEE ALSO
.Xr inet.h 3HEAD ,
.Xr gethostbyname 3NSL ,
.Xr getipnodebyname 3SOCKET ,
.Xr getnetbyname 3SOCKET ,
.Xr hosts 5 ,
.Xr networks 5 ,
.Xr attributes 7
.Sh NOTES
The return value from
.Fn inet_ntoa
points to a buffer which is overwritten on each call.
This buffer is implemented as thread-specific data in multithreaded
applications.
.Pp
IPv4-mapped addresses are not recommended.
.Sh BUGS
The problem of host byte ordering versus network byte ordering is confusing.
.Pp
A simple way to specify Class C network addresses in a manner similar to that
for Class B and Class A is needed.