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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  num-
       bers-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 rightmost two bytes of the binary address.   This
                 notation  is  suitable for specifying (outmoded) Class B net-
                 work 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  rearrange-
                 ment.
       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  deci-
       mal  numbers  is  referred to as IPv4 dotted-decimal notation (or some-
       times: 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 subse-
       quent 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 Inter-
       net 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  compo-
       nents 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)  bound-
       aries.
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) {
               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),  getne-
       tent(3), inet_ntop(3), inet_pton(3), hosts(5), networks(5)
COLOPHON
       This  page  is  part of release 3.53 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                               2013-02-10                           INET(3)