INET(3) BSD Library Functions Manual INET(3)NAME
inet_aton, inet_addr, inet_network, inet_ntoa, inet_ntoa_r, inet_ntop,
inet_pton, inet_makeaddr, inet_lnaof, inet_netof — Internet address
Standard C Library (libc, -lc)
inet_aton(const char *cp, struct in_addr *pin);
inet_addr(const char *cp);
inet_network(const char *cp);
inet_ntoa(struct in_addr in);
inet_ntoa_r(struct in_addr in, char *buf, socklen_t size);
const char *
inet_ntop(int af, const void * restrict src, char * restrict dst,
inet_pton(int af, const char * restrict src, void * restrict dst);
inet_makeaddr(in_addr_t net, in_addr_t lna);
inet_lnaof(struct in_addr in);
inet_netof(struct in_addr in);
The routines inet_aton(), inet_addr() and inet_network() interpret char‐
acter strings representing numbers expressed in the Internet standard ‘.’
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 internal binary representation, in network
byte order). It returns 1 if the address was valid for the specified
address family, or 0 if the address was not 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
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 successfully 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,
The function inet_ntop() converts an address *src 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).
The size argument specifies the size, in bytes, of the buffer *dst.
INET_ADDRSTRLEN and INET6_ADDRSTRLEN define the maximum size required to
convert an address of the respective type. 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. This function is presently valid for
AF_INET and AF_INET6.
The routine inet_ntoa() takes an Internet address and returns an ASCII
string representing the address in ‘.’ notation. The routine
inet_ntoa_r() is the reentrant version of inet_ntoa(). 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 local network address part, respec‐
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 byte order integer values.
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 the VAX the bytes referred to above appear as “d.c.b.a”. That is, VAX
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 speci‐
fying 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, 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).
The constant INADDR_NONE is returned by inet_addr() and inet_network()
for malformed requests.
The inet_ntop() call fails if:
[ENOSPC] size was not large enough to store the presentation
form of the address.
[EAFNOSUPPORT] *src was not an AF_INET or AF_INET6 family address.
SEE ALSObyteorder(3), getaddrinfo(3), gethostbyname(3), getnameinfo(3),
getnetent(3), inet_net(3), hosts(5), networks(5)
IP Version 6 Addressing Architecture, RFC, 2373, July 1998.
The inet_ntop() and inet_pton() functions conform to X/Open Networking
Services Issue 5.2 (“XNS5.2”). Note that inet_pton() does not accept 1-,
2-, or 3-part dotted addresses; all four parts must be specified and are
interpreted only as decimal values. This is a narrower input set than
that accepted by inet_aton().
These functions appeared in 4.2BSD.
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 this problem. The problem of
host byte ordering versus network byte ordering is confusing. The string
returned by inet_ntoa() resides in a static memory area.
The inet_addr() function should return a struct in_addr.
BSD June 14, 2007 BSD