inet_addr, inet_aton, inet_lnaof, inet_makeaddr, inet_netof,
inet_network, inet_ntoa, inet_ntop, inet_pton - Internet address manipulation
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 *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 character
strings representing numbers expressed in the Internet
notation. The inet_pton() function converts a presentation
(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
for the specified address family, or 0 if the address wasn't
the specified address family, or -1 if some system error occurred (in
which case errno will have been set). This function is
for AF_INET and AF_INET6. The inet_aton() routine interprets the specified
character string as an Internet address, placing the
the structure provided. It returns 1 if the string was successfully interpreted,
or 0 if the string was invalid. The inet_addr()
inet_network() functions return numbers suitable for use as
and Internet network numbers, respectively.
The function inet_ntop() converts an address from network
a struct in_addr or some other binary form, in network byte
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
the address in `.' notation. The routine
an Internet network number and a local network address and
Internet address from it. The routines inet_netof() and
break apart Internet host addresses, returning the network
number and local
network address part, respectively.
All Internet addresses are returned in network order (bytes
left to right). All network numbers and local address parts
as machine format integer values.
INTERNET ADDRESSES (IP VERSION 4) [Toc] [Back]
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
486 and Pentium processors) the bytes referred to above appear as
``d.c.b.a''. That is, little-endian bytes are ordered from
When a three part address is specified, the last part is interpreted as a
16-bit quantity and placed in the rightmost two bytes of the
This makes the three part address format convenient
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 rightmost three bytes of
address. This makes the two part address format convenient
Class A network addresses as ``net.host''.
When only one part is given, the value is stored directly in
address without any byte rearrangement.
All numbers supplied as ``parts'' in a `.' notation may be
or hexadecimal, as specified in the C language (i.e., a
or 0X implies hexadecimal; otherwise, a leading 0 implies
the number is interpreted as decimal).
INTERNET ADDRESSES (IP VERSION 6) [Toc] [Back]
In order to support scoped IPv6 addresses, getaddrinfo(3)
getnameinfo(3) are recommended rather than the functions
The presentation format of an IPv6 address is given in RFC
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. Examples:
Note that it is not necessary to write the leading zeros in an individual
field, but there must be at least one numeral in
(except for the case described in 2.).
2. Due to the method of allocating certain styles of IPv6
will be common for addresses to contain long strings of
In order to make writing addresses containing zero bits
special syntax is available to compress the zeros. The
``::'' indicates multiple groups of 16 bits of zeros.
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
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
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
for malformed requests.
byteorder(3), gethostbyname(3), getnetent(3), inet_net(3),
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 the IETF
IPv6 BSD API
and address formatting specifications. Note that inet_pton
does not accept
1-, 2-, or 3-part dotted addresses; all four parts must
This is a narrower input set than that accepted by
The inet_addr, inet_network, inet_makeaddr, inet_lnaof and
functions appeared in 4.2BSD. The inet_aton and inet_ntoa
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
inet_addr() should have been designed to return a struct
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 confusing.
The string returned by inet_ntoa() resides in a static memory area.
OpenBSD 3.6 June 18, 1997
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