NAME
inet_addr,
inet_aton,
inet_lnaof,
inet_makeaddr,
inet_netof,
inet_network,
inet_ntoa,
inet_ntop,
inet_pton,
addr,
ntoa,
network —
Internet address
manipulation routines
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <arpa/inet.h>
in_addr_t
inet_addr(
const
char *cp);
int
inet_aton(
const
char *cp,
struct in_addr
*addr);
in_addr_t
inet_lnaof(
struct
in_addr in);
struct in_addr
inet_makeaddr(
in_addr_t
net,
in_addr_t lna);
in_addr_t
inet_netof(
struct
in_addr in);
in_addr_t
inet_network(
const
char *cp);
char *
inet_ntoa(
struct
in_addr in);
const char *
inet_ntop(
int
af,
const void * restrict
src,
char * restrict
dst,
socklen_t size);
int
inet_pton(
int
af,
const char * restrict
src,
void * restrict
dst);
DESCRIPTION
The routines
inet_aton(),
inet_addr() and
inet_network() interpret character strings representing
numbers expressed in the Internet standard “dotted quad” notation.
The
inet_pton() function converts a presentation format
address (that is, printable form as held in a character string) to network
format (usually a
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
errno will have been set). This function is
presently valid for
AF_INET
and
AF_INET6
.
The
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.
The
inet_addr() and
inet_network() functions
return numbers suitable for use as Internet addresses and Internet network
numbers, respectively.
The function
inet_ntop() converts an address from network
format (usually a
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,
errno will have been set), or it returns a pointer to
the destination string. The
size parameter is the size
of the
buf argument.
The routine
inet_ntoa() takes an Internet address and returns
an ASCII string representing the address in “dotted quad”
notation.
The routine
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
struct in_addr form that value should be passed as the
first parameter and ‘
0L
’ should be given
for the second parameter.
The routines
inet_netof() and
inet_lnaof()
break apart Internet host addresses, returning the network number and local
network address part, respectively (both in host order).
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.
INTERNET ADDRESSES (IP
VERSION 4)
Values specified using the “dotted quad” notation take one of the
following forms:
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. Intel i386, i486 and Pentium
processors) the bytes referred to above appear as
“
d.c.b.a
”. That is, little-endian bytes
are ordered from right to left.
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 “
128.net.host
”.
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 “
net.host
”.
When only one part is given, the value is stored directly in the network address
without any byte rearrangement.
All numbers supplied as “parts” in a “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).
INTERNET ADDRESSES (IP
VERSION 6)
In order to support scoped IPv6 addresses, the use of
getaddrinfo(3) and
getnameinfo(3) is
recommended rather than the functions presented here.
The presentation format of an IPv6 address is given in RFC 2373:
There are three conventional forms for representing IPv6 addresses as text
strings:
- 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:
FEDC:BA98:7654:3210:FEDC:BA98:7654:3210
1080:0:0:0:8:800:200C:417A
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).
- 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.
For example the following addresses:
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
may be represented as:
1080::8:800:200C:417A a unicast address
FF01::43 a multicast address
::1 the loopback address
:: the unspecified addresses
- 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:
0:0:0:0:0:0:13.1.68.3
0:0:0:0:0:FFFF:129.144.52.38
or in compressed form:
::13.1.68.3
::FFFF:129.144.52.38
DIAGNOSTICS
The constant
INADDR_NONE
is returned by
inet_addr() and
inet_network() for
malformed requests.
ERRORS
The
inet_ntop() and
inet_pton() functions
may fail with
-
-
- [
EAFNOSUPPORT
]
- The value of af was not
AF_INET
or AF_INET6
.
The
inet_ntop() function may fail with
-
-
- [
ENOSPC
]
- The size indicated for
dst was too small to store the presentation form of
the network address.
SEE ALSO
byteorder(3),
gethostbyname(3),
getnetent(3),
inet_net(3),
hosts(5),
networks(5)
IP Version 6 Addressing Architecture,
RFC 2373, July 1998.
Basic Socket Interface Extensions for
IPv6, RFC 3493, February
2003.
STANDARDS
The
inet_ntop() and
inet_pton() functions
conform to
IEEE Std 1003.1-2001
(“POSIX.1”). Note that
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
inet_aton().
HISTORY
The
inet_addr(),
inet_network(),
inet_makeaddr(),
inet_lnaof() and
inet_netof() functions appeared in
4.2BSD. They were changed to use
in_addr_t in place of
unsigned
long in
NetBSD 2.0. The
inet_aton() and
inet_ntoa() functions
appeared in
4.3BSD. The
inet_pton()
and
inet_ntop() functions appeared in BIND 4.9.4 and thence
NetBSD 1.3; they were also in
X/Open
Networking Services Issue 5.2 (“XNS5.2”).
BUGS
The value
INADDR_NONE
(0xffffffff) is a valid broadcast
address, but
inet_addr() cannot return that value without
indicating failure. The newer
inet_aton() function does not
share this problem.
The problem of host byte ordering versus network byte ordering is confusing.
The string returned by
inet_ntoa() resides in a static memory
area.
The function
inet_addr() should return a
struct in_addr.
The function
inet_network() does not support byte
rearrangement for one, two, and three part addresses.