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           #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


           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-
           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
           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;


           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


           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
           Here are some example runs:
               $ ./a.out      # Last byte is in octal
               $ ./a.out 0x7f.1               # First byte is in hex
       Program source
           #define _BSD_SOURCE
           #include <arpa/inet.h>
           #include <stdio.h>
           #include <stdlib.h>
           main(int argc, char *argv[])
               struct in_addr addr;
               if (argc != 2) {
                   fprintf(stderr, "%s <dotted-address>\n", argv[0]);
               if (inet_aton(argv[1], &addr) == 0) {
                   fprintf(stderr, "Invalid address\n");
               printf("%s\n", inet_ntoa(addr));


           byteorder(3), getaddrinfo(3), gethostbyname(3), getnameinfo(3),  getne-
           tent(3), inet_ntop(3), inet_pton(3), hosts(5), networks(5)

    GNU 2013-02-10 INET(3)


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