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(in_addr_t net, in_addr_t 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  sup‐
       plied 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 A 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).

       inet_aton() returns 1 if the supplied string was successfully interpreted,  or  0  if  the
       string is invalid (errno is not set on error).

       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 getad‐
       drinfo(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  stati‐
       cally 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  sig‐
                 nificant 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  arbi‐
       trary bit (rather than byte) boundaries.

EXAMPLE
       An  example of the use of inet_aton() and inet_ntoa() is shown below.  Here are some exam‐
       ple 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) {
               fprintf(stderr, "Invalid address\n");
               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_net_pton(3), inet_ntop(3), inet_pton(3), hosts(5), networks(5)

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.



GNU                                         2014-05-28                                    INET(3)