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inet (Internet address manipulation routines)
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

inet_aton(const char *cp, struct in_addr *pin);

inet_addr(const char *cp);

inet_network(const char *cp);

char *
inet_ntoa(struct in_addr in);

char *
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, socklen_t size);

inet_pton(int af, const char * restrict src, void * restrict dst);

struct in_addr
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 character strings representing numbers expressed in the Internet standard "." 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 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 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 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, respectively.

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, respectively.

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.

Internet Addresses

Values specified using the '.' notation take one of the following forms:


When four parts are specified, each is interpretedas 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 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 right most 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 '.' 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 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 .

Return Values

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.


Versions Link to
INtime 4.0 netlib.lib

See Also

RFC 2373 July 1998 "IP Version 6 Addressing Architecture"

See Also