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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 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 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 inter-
                 preted 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
       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  nota-
       tion,  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 signif-
                 icant  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 sig-
                 nificant 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 compo-
       nents 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 information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



GNU                               2008-06-19                           INET(3)

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