Internet Protocol version 4 (IPv4)
address manipulation routines
char *cp, struct in_addr
The functions presented here only support IPv4 addresses. In order to support IPv6 addresses as well, inet_ntop(3) and inet_pton(3) should be used rather than the functions presented here. Scoped IPv6 addresses are supported via getaddrinfo(3) and getnameinfo(3).
inet_network() interpret character strings
representing numbers expressed in the Internet standard “dot”
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 was invalid.
functions return numbers suitable for use as Internet addresses and Internet
network numbers, respectively. Both functions return the constant
INADDR_NONE if the specified character string is
takes an Internet address and returns an ASCII string representing the
address in dot notation.
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 dot notation take one of the following forms:
a.b.c.d a.b.c a.b a
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 rightmost two bytes of
the network address. This makes the three part address format convenient for
specifying Class B network addresses as
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 the network
address. This makes the two part address format convenient for specifying
Class A network addresses as
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 dot notation may be decimal, octal, or hexadecimal, as specified in the C language (i.e., a leading 0x or 0X implies hexadecimal; a leading 0 implies octal; otherwise, the number is interpreted as decimal).
byteorder(3), gethostbyname(3), getnetent(3), inet_lnaof(3), inet_net(3), inet_ntop(3), hosts(5), networks(5)
inet_aton functions conform to IEEE
Std 1003.1-2008 (“POSIX.1”).
inet_network functions appeared in
inet_ntoa functions appeared in
INADDR_NONE (0xffffffff) is a
valid broadcast address, but
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
The problem of host byte ordering versus network byte ordering is confusing.
The string returned by
in a static memory area.