INET(3) BSD 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 repre-
senting 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 inter-
nal 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 success-
fully 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
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:
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 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 envi-
ronment of IPv4 and IPv6 nodes is x:x:x:x:x:x:d.d.d.d, where the 'x's are the hexadeci-
mal 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 repre-
or in compressed form:
The constant INADDR_NONE is returned by inet_addr() and inet_network() for malformed
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.
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
The problem of host byte ordering versus network byte ordering is confusing.
The string returned by inet_ntoa() resides in a static memory area.
inet_addr() should return a struct in_addr.
BSD September 22, 2011 BSD