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INET(3) OpenBSD Programmer's Manual INET(3)
inet_addr, inet_aton, inet_lnaof, inet_makeaddr, inet_netof,
inet_network, inet_ntoa, inet_ntop, inet_pton - Internet address manipu-
inet_addr(const char *cp);
inet_aton(const char *cp, struct in_addr *addr);
inet_lnaof(struct in_addr in);
inet_makeaddr(unsigned long net, unsigned long 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 *src, char *dst, size_t size);
inet_pton(int af, const char *src, void *dst);
The routines inet_aton(), inet_addr() and inet_network() interpret char-
acter strings representing numbers expressed in the Internet standard `.'
notation. The inet_pton() function converts a presentation format ad-
dress (that is, printable form as held in a character string) to network
format (usually a struct in_addr or some other internal binary represen-
tation, in network byte order). It returns 1 if the address was valid
for the specified address family, or 0 if the address wasn't parseable 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 speci-
fied character string as an Internet address, placing the address into
the structure provided. It returns 1 if the string was successfully in-
terpreted, or 0 if the string is invalid. The inet_addr() and
inet_network() functions return numbers suitable for use as Internet ad-
dresses 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 routine
inet_ntoa() takes an Internet address and returns an ASCII string repre-
senting the address in `.' notation. The routine inet_makeaddr() takes
an Internet network number and a local network address and constructs an
Internet address from it. The routines inet_netof() and inet_lnaof()
break apart Internet host addresses, returning the network number and lo-
cal network address part, respectively.
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 `.' 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 (such as the Intel 386,
486 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 ad-
dress. This makes the three part address format convenient for specify-
ing 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 specify-
ing 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 `.' notation may be decimal, oc-
tal, or hexadecimal, as specified in the C language (i.e., a leading 0x
or 0X implies hexadecimal; otherwise, a leading 0 implies octal; other-
wise, the number is interpreted as decimal).
INTERNET ADDRESSES (IP VERSION 6)
The presentation format of an IPv6 address is given in [RFC1884 2.2]:
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 hex-
adecimal values of the eight 16-bit pieces of the address. Exam-
Note that it is not necessary to write the leading zeros in an indi-
vidual 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 com-
press 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 ze-
ros 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),
The inet_ntop and inet_pton functions conforms to the IETF IPng BSD API
and address formatting specifications. Note that inet_pton does not ac-
cept 1-, 2-, or 3-part dotted addresses; all four parts must be speci-
fied. 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. The inet_aton and inet_ntoa functions ap-
peared in 4.3BSD. The inet_pton and inet_ntop functions appeared in BIND
The value INADDR_NONE (0xffffffff) is a valid broadcast address, but
inet_addr() cannot return that value without indicating failure. Also,
inet_addr() should have been designed to return a struct in_addr. The
newer inet_aton() function does not share these problems, and almost all
existing code should be modified to use inet_aton() instead.
The problem of host byte ordering versus network byte ordering is confus-
The string returned by inet_ntoa() resides in a static memory area.
OpenBSD 2.6 June 18, 1997 3
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.
(Corrections, notes, and links courtesy of RocketAware.com)
FreeBSD Sources for inet(3) functions
OpenBSD sources for inet(3)
Up to: Host, service name, and address operations - Methods and functions for doing address, host, user, and service name lookups (DNS). also Internet Assigned Numbers
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