INET(3) 						     Linux Programmer's Manual							   INET(3)

NAME

       inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr, inet_lnaof, inet_netof - Internet address manipulation routines

SYNOPSIS

       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <arpa/inet.h>

       int inet_aton(const char *cp, struct in_addr *inp);

       in_addr_t inet_addr(const char *cp);

       in_addr_t inet_network(const char *cp);

       char *inet_ntoa(struct in_addr in);

       struct in_addr inet_makeaddr(int net, int host);

       in_addr_t inet_lnaof(struct in_addr in);

       in_addr_t inet_netof(struct in_addr in);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       inet_aton(), inet_ntoa(): _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

       inet_aton()  converts  the  Internet  host  address cp from the IPv4 numbers-and-dots notation into binary form (in network byte order) and
       stores it in the structure that inp points to.  inet_aton() returns nonzero if the address is valid, zero if not.  The address supplied	in
       cp can have one of the following forms:

       a.b.c.d	 Each  of  the	four  numeric  parts specifies a byte of the address; the bytes are assigned in left-to-right order to produce the
		 binary address.

       a.b.c	 Parts a and b specify the first two bytes of the binary address.  Part c is interpreted as a 16-bit value that defines the right-
		 most two bytes of the binary address.	This notation is suitable for specifying (outmoded) Class B network addresses.

       a.b	 Part  a specifies the first byte of the binary address.  Part b is interpreted as a 24-bit value that defines the rightmost three
		 bytes of the binary address.  This notation is suitable for specifying (outmoded) Class C network addresses.

       a	 The value a is interpreted as a 32-bit value that is stored directly into the binary address without any byte rearrangement.

       In all of the above forms, components of the dotted address can be specified in decimal, octal (with a leading 0), or hexadecimal,  with  a
       leading 0X).  Addresses in any of these forms are collectively termed IPV4 numbers-and-dots notation.  The form that uses exactly four dec-
       imal numbers is referred to as IPv4 dotted-decimal notation (or sometimes: IPv4 dotted-quad notation).

       The inet_addr() function converts the Internet host address cp from IPv4 numbers-and-dots notation into binary data in network byte  order.
       If  the	input  is  invalid,  INADDR_NONE  (usually  -1)  is  returned.	 Use of this function is problematic because -1 is a valid address
       (255.255.255.255).  Avoid its use in favor of inet_aton(), inet_pton(3), or getaddrinfo(3) which provide a cleaner way  to  indicate  error
       return.

       The  inet_network()  function converts cp, a string in IPv4 numbers-and-dots notation, into a number in host byte order suitable for use as
       an Internet network address.  On success, the converted address is returned.  If the input is invalid, -1 is returned.

       The inet_ntoa() function converts the Internet host address in, given in network byte order, to a string in IPv4  dotted-decimal  notation.
       The string is returned in a statically allocated buffer, which subsequent calls will overwrite.

       The inet_lnaof() function returns the local network address part of the Internet address in.  The returned value is in host byte order.

       The inet_netof() function returns the network number part of the Internet address in.  The returned value is in host byte order.

       The  inet_makeaddr() function is the converse of inet_netof() and inet_lnaof().	It returns an Internet host address in network byte order,
       created by combining the network number net with the local address host, both in host byte order.

       The structure in_addr as used in inet_ntoa(), inet_makeaddr(), inet_lnaof() and inet_netof() is defined in <netinet/in.h> as:

	   typedef uint32_t in_addr_t;

	   struct in_addr {
	       in_addr_t s_addr;
	   };

CONFORMING TO

       4.3BSD.	inet_addr() and inet_ntoa() are specified in POSIX.1-2001.  inet_aton() is not specified in POSIX.1-2001, but is available on most
       systems.

NOTES

       On  the	i386 the host byte order is Least Significant Byte first (little endian), whereas the network byte order, as used on the Internet,
       is Most Significant Byte first (big endian).

       inet_lnaof(), inet_netof(), and inet_makeaddr() are legacy functions that assume they are dealing with classful network addresses.   Class-
       ful networking divides IPv4 network addresses into host and network components at byte boundaries, as follows:

       Class A	 This  address	type  is  indicated by the value 0 in the most significant bit of the (network byte ordered) address.  The network
		 address is contained in the most significant byte, and the host address occupies the remaining three bytes.

       Class B	 This address type is indicated by the binary value 10 in the most significant two bits of the address.  The  network  address	is
		 contained in the two most significant bytes, and the host address occupies the remaining two bytes.

       Class C	 This address type is indicated by the binary value 110 in the most significant three bits of the address.  The network address is
		 contained in the three most significant bytes, and the host address occupies the remaining byte.

       Classful network addresses are now obsolete, having been superseded by Classless Inter-Domain Routing (CIDR), which divides addresses  into
       network and host components at arbitrary bit (rather than byte) boundaries.

EXAMPLE

       An example of the use of inet_aton() and inet_ntoa() is shown below.  Here are some example runs:

	   $ ./a.out 226.000.000.037	  # Last byte is in octal
	   226.0.0.31
	   $ ./a.out 0x7f.1		  # First byte is in hex
	   127.0.0.1

   Program source

       #define _BSD_SOURCE
       #include <arpa/inet.h>
       #include <stdio.h>
       #include <stdlib.h>

       int
       main(int argc, char *argv[])
       {
	   struct in_addr addr;

	   if (argc != 2) {
	       fprintf(stderr, "%s <dotted-address>
", argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   if (inet_aton(argv[1], &addr) == 0) {
	       perror("inet_aton");
	       exit(EXIT_FAILURE);
	   }

	   printf("%s
", inet_ntoa(addr));
	   exit(EXIT_SUCCESS);
       }

SEE ALSO

       byteorder(3), getaddrinfo(3), gethostbyname(3), getnameinfo(3), getnetent(3), inet_ntop(3), inet_pton(3), hosts(5), networks(5)

COLOPHON

       This  page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2008-06-19								   INET(3)