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.Dd $Mdocdate: September 1 2021 $
.Dt INET_NTOP 3
.Os
.Sh NAME
.Nm inet_ntop ,
.Nm inet_pton
.Nd convert Internet addresses between presentation and network formats
.Sh SYNOPSIS
.In sys/socket.h
.In arpa/inet.h
.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
.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 .
.Pp
The function
.Fn inet_ntop
converts an address from network format to presentation format.
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.
.Pp
All Internet addresses are returned in network
order (bytes ordered from left to right).
.Sh INTERNET ADDRESSES (IP VERSION 4)
Values must be specified using the standard dot notation:
.Bd -literal -offset indent
a.b.c.d
.Ed
.Pp
All four parts must be decimal numbers between 0 and 255, inclusive,
and are 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.
.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 4291:
.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 SEE ALSO
.Xr gethostbyname 3 ,
.Xr inet_addr 3 ,
.Xr inet_net_ntop 3 ,
.Xr hosts 5
.Sh STANDARDS
The
.Nm inet_ntop
and
.Nm inet_pton
functions conform to the IETF IPv6 BSD API and address formatting
specifications, as well as
.St -p1003.1-2008 .
.Sh HISTORY
The
.Nm inet_pton
and
.Nm inet_ntop
functions appeared in BIND 4.9.4.
.Sh CAVEATS
Note that
.Nm inet_pton
does not accept 1-, 2-, or 3-part dotted addresses;
all four parts must be specified and must be in decimal
(and not octal or hexadecimal).
This is a narrower input set than that accepted by
.Nm inet_aton .
.Pp
.Rs
.%A R. Gilligan
.%A S. Thomson
.%A J. Bound
.%A J. McCann
.%A W. Stevens
.%D February 2003
.%R RFC 3493
.%T Basic Socket Interface Extensions for IPv6
.Re
.Pp
.Rs
.%A R. Hinden
.%A S. Deering
.%D February 2006
.%R RFC 4291
.%T IP Version 6 Addressing Architecture
.Re