INET(3)                   OpenBSD Programmer's Manual                  INET(3)


     inet_aton, inet_addr, inet_network, inet_pton, inet_ntop, inet_ntoa,
     inet_makeaddr, inet_netof, inet_lnaof - Internet address manipulation


     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <arpa/inet.h>

     inet_aton(const char *cp, struct in_addr *addr);

     inet_addr(const char *cp);

     inet_network(const char *cp);

     inet_pton(int af, const char *src, void *dst);

     const char *
     inet_ntop(int af, const void *src, char *dst, socklen_t size);

     char *
     inet_ntoa(struct in_addr in);

     struct in_addr
     inet_makeaddr(in_addr_t net, in_addr_t lna);

     inet_netof(struct in_addr in);

     inet_lnaof(struct in_addr in);


     The routines inet_aton(), inet_addr(), and inet_network() interpret
     character strings representing numbers expressed in the Internet standard
     ``dot'' notation.

     The inet_aton() routine interprets the specified character string as an
     Internet address, placing the address into the structure provided.  It
     returns 1 if the string was successfully interpreted, or 0 if the string
     was invalid.

     The inet_addr() and inet_network() functions return numbers suitable for
     use as Internet addresses and Internet network numbers, respectively.
     Both functions return the constant INADDR_NONE if the specified character
     string is malformed.

     The inet_pton() function converts a presentation format address (that is,
     printable form as held in a character string) to network format (usually
     a struct in_addr or some other internal binary representation, in network
     byte order).  It returns 1 if the address was valid for the specified
     address family; 0 if the address wasn't parseable in the specified
     address family; or -1 if some system error occurred (in which case errno
     will have been set).  This function is presently valid for AF_INET and

     The function inet_ntop() converts an address from network format (usually
     a struct in_addr or some other binary form, in network byte order) to
     presentation format (suitable for external display purposes).  It returns
     NULL if a system error occurs (in which case, errno will have been set),
     or it returns a pointer to the destination string.

     The routine inet_ntoa() takes an Internet address and returns an ASCII
     string representing the address in dot notation.

     The routine inet_makeaddr() takes an Internet network number and a local
     network address and constructs an Internet address from it.

     The routines inet_netof() and inet_lnaof() break apart Internet host
     addresses, returning the network number and local network address part,

     All Internet addresses are returned in network order (bytes ordered from
     left to right).  All network numbers and local address parts are returned
     as machine format integer values.


     Values specified using dot notation take one of the following forms:


     When four parts are specified, each is interpreted as a byte of data and
     assigned, from left to right, to the four bytes of an Internet address.
     Note that when an Internet address is viewed as a 32-bit integer quantity
     on a system that uses little-endian byte order (such as the Intel 386,
     486 and Pentium processors) the bytes referred to above appear as
     ``d.c.b.a''.  That is, little-endian bytes are ordered from right to

     When a three part address is specified, the last part is interpreted as a
     16-bit quantity and placed in the rightmost two bytes of the network
     address.  This makes the three part address format convenient for
     specifying Class B network addresses as ``''.

     When a two part address is supplied, the last part is interpreted as a
     24-bit quantity and placed in the rightmost three bytes of the network
     address.  This makes the two part address format convenient for
     specifying Class A network addresses as ``''.

     When only one part is given, the value is stored directly in the network
     address without any byte rearrangement.

     All numbers supplied as ``parts'' in a dot notation may be decimal,
     octal, or hexadecimal, as specified in the C language (i.e., a leading 0x
     or 0X implies hexadecimal; a leading 0 implies octal; otherwise, the
     number is interpreted as decimal).


     In order to support scoped IPv6 addresses, getaddrinfo(3) and
     getnameinfo(3) are recommended rather than the functions presented here.

     The presentation format of an IPv6 address is given in RFC 4291:

     There are three conventional forms for representing IPv6 addresses as
     text strings:

     1.   The preferred form is x:x:x:x:x:x:x:x, where the 'x's are the
          hexadecimal values of the eight 16-bit pieces of the address.


          Note that it is not necessary to write the leading zeros in an
          individual field, but there must be at least one numeral in every
          field (except for the case described in 2.).

     2.   Due to the method of allocating certain styles of IPv6 addresses, it
          will be common for addresses to contain long strings of zero bits.
          In order to make writing addresses containing zero bits easier, a
          special syntax is available to compress the zeros.  The use of
          ``::'' indicates multiple groups of 16 bits of zeros.  The ``::''
          can only appear once in an address.  The ``::'' can also be used to
          compress the leading and/or trailing zeros in an address.

          For example the following addresses:

                1080:0:0:0:8:800:200C:417A  a unicast address
                FF01:0:0:0:0:0:0:43         a multicast address
                0:0:0:0:0:0:0:1             the loopback address
                0:0:0:0:0:0:0:0             the unspecified addresses

          may be represented as:

                1080::8:800:200C:417A       a unicast address
                FF01::43                    a multicast address
                ::1                         the loopback address
                ::                          the unspecified addresses

     3.   An alternative form that is sometimes more convenient when dealing
          with a mixed environment of IPv4 and IPv6 nodes is
          x:x:x:x:x:x:d.d.d.d, where the 'x's are the hexadecimal values of
          the six high-order 16-bit pieces of the address, and the 'd's are
          the decimal values of the four low-order 8-bit pieces of the address
          (standard IPv4 representation).  Examples:


          or in compressed form:



     byteorder(3), gethostbyname(3), getnetent(3), inet_net(3), hosts(5),


     The inet_ntop and inet_pton functions conform to the IETF IPv6 BSD API
     and address formatting specifications.  Note that inet_pton does not
     accept 1-, 2-, or 3-part dotted addresses; all four parts must be
     specified.  This is a narrower input set than that accepted by inet_aton.

     R. Gilligan, S. Thomson, J. Bound, J. McCann, and W. Stevens, Basic
     Socket Interface Extensions for IPv6, RFC 3493, February 2003.

     R. Hinden and S. Deering, IP Version 6 Addressing Architecture, RFC 4291,
     February 2006.


     The inet_addr, inet_network, inet_makeaddr, inet_lnaof, and inet_netof
     functions appeared in 4.2BSD.  The inet_aton and inet_ntoa functions
     appeared in 4.3BSD.  The inet_pton and inet_ntop functions appeared in
     BIND 4.9.4.


     The value INADDR_NONE (0xffffffff) is a valid broadcast address, but
     inet_addr() cannot return that value without indicating failure.  Also,
     inet_addr() should have been designed to return a struct in_addr.  The
     newer inet_aton() function does not share these problems, and almost all
     existing code should be modified to use inet_aton() instead.

     The problem of host byte ordering versus network byte ordering is

     The string returned by inet_ntoa() resides in a static memory area.

OpenBSD 5.4                      June 5, 2013                      OpenBSD 5.4

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