inet

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  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 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  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-

       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;
           };

ATTRIBUTES
       For   an   explanation   of   the  terms  used  in  this  section,  see
       attributes(7).

       +-------------------------------+---------------+----------------+
       |Interface                      | Attribute     | Value          |
       +-------------------------------+---------------+----------------+
       |inet_aton(), inet_addr(),      | Thread safety | MT-Safe locale |
       |inet_network(), inet_ntoa()    |               |                |
       +-------------------------------+---------------+----------------+
       |inet_makeaddr(), inet_lnaof(), | Thread safety | MT-Safe        |
       |inet_netof()                   |               |                |
       +-------------------------------+---------------+----------------+
CONFORMING TO
       inet_addr(), inet_ntoa(): POSIX.1-2001, POSIX.1-2008, 4.3BSD.

       inet_aton() is not specified in POSIX.1, but is available on most  sys-
       tems.

NOTES
       On  x86  architectures,  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).
                 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_net_pton(3), inet_ntop(3), inet_pton(3),  hosts(5),  net-
       works(5)
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