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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 manipu-
       lation 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 non-zero 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 rightmost 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 decimal numbers is referred to as IPv4 dotted-decimal nota-
       tion (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  func-
       tion  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.  Classful 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>\n", argv[0]);
               exit(EXIT_FAILURE);
           }

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

           printf("%s\n", 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2008-06-19                           INET(3)