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 rou-
       tines

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 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 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 notation (or sometimes:  IPv4  dot-
       ted-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 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),  net-
       works(5)

COLOPHON
       This  page  is  part of release 3.25 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                                                        2008-06-19                                                    INET(3)