inet, inet6, inet_ntop, inet_pton, inet_aton, inet_addr, inet_network,
inet_makeaddr, inet_lnaof, inet_netof, inet_ntoa - Internet address
cc [ flag... ] file... -lsocket -lnsl [ library... ]
const char *inet_ntop(int af, const void *addr, char *cp,
int inet_pton(int af, const char *cp, void *addr);
int inet_aton(const char *cp, struct in_addr *addr);
in_addr_t inet_addr(const char *cp);
in_addr_t inet_network(const char *cp);
struct in_addr inet_makeaddr(const int net, const int lna);
int inet_lnaof(const struct in_addr in);
int inet_netof(const struct in_addr in);
char *inet_ntoa(const struct in_addr in);
The inet_ntop() and inet_pton() functions can manipulate both IPv4 and
IPv6 addresses. The inet_aton(), inet_addr(), inet_network(),
inet_makeaddr(), inet_lnaof(), inet_netof(), and inet_ntoa() functions
can only manipulate IPv4 addresses.
The inet_ntop() function converts a numeric address into a string suit‐
able for presentation. The af argument specifies the family of the
address which can be AF_INET or AF_INET6. The addr argument points to a
buffer that holds an IPv4 address if the af argument is AF_INET. The
addr argument points to a buffer that holds an IPv6 address if the af
argument is AF_INET6. The address must be in network byte order. The cp
argument points to a buffer where the function stores the resulting
string. The application must specify a non-NULL cp argument. The size
argument specifies the size of this buffer. For IPv6 addresses, the
buffer must be at least 46-octets. For IPv4 addresses, the buffer must
be at least 16-octets. To allow applications to easily declare buffers
of the proper size to store IPv4 and IPv6 addresses in string form, the
following two constants are defined in <netinet/in.h>:
#define INET_ADDRSTRLEN 16
#define INET6_ADDRSTRLEN 46
The inet_pton() function converts the standard text presentation form
of a function to the numeric binary form. The af argument specifies the
family of the address. Currently, the AF_INET and AF_INET6 address fam‐
ilies are supported. The cp argument points to the string being passed
in. The addr argument points to a buffer where the function stores the
numeric address. The calling application must ensure that the buffer
referred to by addr is large enough to hold the numeric address, at
least 4 bytes for AF_INET or 16 bytes for AF_INET6.
The inet_aton(), inet_addr(), and inet_network() functions interpret
character strings that represent numbers expressed in the IPv4 standard
'.' notation, returning numbers suitable for use as IPv4 addresses and
IPv4 network numbers, respectively. The inet_makeaddr() function uses
an IPv4 network number and a local network address to construct an IPv4
address. The inet_netof() and inet_lnaof() functions break apart IPv4
host addresses, then return the network number and local network
The inet_ntoa() function returns a pointer to a string in the base 256
notation d.d.d.d. See the following section on IPv4 addresses.
Internet addresses are returned in network order, bytes ordered from
left to right. Network numbers and local address parts are returned as
machine format integer values.
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. For example:
It is not necessary to write the leading zeros in an indi‐
vidual field. There must be at least one numeral in every
field, except when the special syntax described in the fol‐
lowing is used.
2. It is common for addresses to contain long strings of zero
bits in some methods used to allocate certain IPv6 address
styles. A special syntax is available to compress the zeros.
The use of "::" indicates multiple groups of 16 bits of
zeros. The :: may only appear once in an address. The ::
can also be used to compress the leading and trailing zeros
in an address. For example:
3. The alternative form x:x:x:x:x:x:d.d.d.d is sometimes more
convenient when dealing with a mixed environment of IPv4 and
IPv6 nodes. The x's in this form represent the hexadecimal
values of the six high-order 16-bit pieces of the address.
The d's represent the decimal values of the four low-order
8-bit pieces of the standard IPv4 address. For example:
The ::FFFF:d.d.d.d and ::d.d.d.d pieces are the general
forms of an IPv4-mapped IPv6 address and an IPv4-compatible
The IPv4 portion must be in the d.d.d.d form. The following
forms are invalid:
The ::FFFF:d form is a valid but unconventional representa‐
tion of the IPv4-compatible IPv6 address ::255.255.0.d.
The ::d form corresponds to the general IPv6 address
Values specified using `.' notation take one of the following forms:
When four parts are specified, each part is interpreted as a byte of
data and assigned from left to right to the four bytes of an IPv4
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. The three part address format is convenient for specifying
Class B network addresses such 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. The two part address format is convenient for specifying Class
A network addresses such as net.host.
When only one part is given, the value is stored directly in the net‐
work address without any byte rearrangement.
With the exception of inet_pton(), numbers supplied as parts in '.'
notation may be decimal, octal, or hexadecimal, as specified in C lan‐
guage. For example, a leading 0x or 0X implies hexadecimal. A leading 0
implies octal. Otherwise, the number is interpreted as decimal.
For IPv4 addresses, inet_pton() accepts only a string in standard IPv4
Each number has one to three digits with a decimal value between 0 and
The inet_addr() function has been obsoleted by inet_aton().
The inet_aton() function returns nonzero if the address is valid, 0 if
the address is invalid.
The inet_ntop() function returns a pointer to the buffer that contains
a string if the conversion succeeds. Otherwise, NULL is returned. Upon
failure, errno is set to EAFNOSUPPORT if the af argument is invalid or
ENOSPC if the size of the result buffer is inadequate.
The inet_pton() function returns 1 if the conversion succeeds, 0 if the
input is not a valid IPv4 dotted-decimal string or a valid IPv6
address string. The function returns -1 with errno set to EAFNOSUPPORT
if the af argument is unknown.
The value INADDR_NONE, which is equivalent to (in_addr_t)(-1), is
returned by inet_addr() and inet_network() for malformed requests.
The functions inet_netof() and inet_lnaof() break apart IPv4 host
addresses, returning the network number and local network address part,
The function inet_ntoa() returns a pointer to a string in the base 256
notation d.d.d.d, described in the section on IPv4 addresses.
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ See below. │
│MT-Level │ Safe │
The inet_ntop(), inet_pton(), inet_aton(), inet_addr(), and inet_net‐
work() functions are Committed. The inet_lnaof(), inet_makeaddr(),
inet_netof(), and inet_network() functions are Committed (Obsolete).
SEE ALSOgethostbyname(3NSL), getipnodebyname(3SOCKET), getnetbyname(3SOCKET),
inet.h(3HEAD), hosts(4), networks(4), attributes(5)NOTES
The return value from inet_ntoa() points to a buffer which is overwrit‐
ten on each call. This buffer is implemented as thread-specific data in
IPv4-mapped addresses are not recommended.
The problem of host byte ordering versus network byte ordering is con‐
fusing. A simple way to specify Class C network addresses in a manner
similar to that for Class B and Class A is needed.
Nov 28, 2007 INET(3SOCKET)