INET(3) Library Functions Manual INET(3)
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
Standard C Library (libc, -lc)
inet_addr(const char *cp);
inet_aton(const char *cp, struct in_addr *addr);
inet_lnaof(struct in_addr in);
inet_makeaddr(in_addr_t net, in_addr_t lna);
inet_netof(struct in_addr in);
inet_network(const char *cp);
inet_ntoa(struct in_addr in);
const char *
inet_ntop(int af, const void * restrict src, char * restrict
dst, socklen_t size);
inet_pton(int af, const char * restrict src, void * restrict dst);
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
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
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
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
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
1. 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.
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).
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
3. 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:
or in compressed form:
The constant INADDR_NONE is returned by inet_addr() and inet_network()
for malformed requests.
byteorder(3), gethostbyname(3), getnetent(3), inet_net(3), hosts(5),
IP Version 6 Addressing Architecture, RFC 2373, July 1998.
Basic Socket Interface Extensions for IPv6, RFC 3493, February 2003.
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. This is a narrower
input set than that accepted by inet_aton.
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'').
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
The string returned by inet_ntoa() resides in a static memory area.
inet_addr() should return a struct in_addr.
NetBSD 6.1.5 September 22, 2011 NetBSD 6.1.5