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NAME    [Toc]    [Back]

     inet_addr, inet_aton, inet_lnaof, inet_makeaddr, inet_netof,
     inet_network, inet_ntoa, inet_ntop, inet_pton, addr, ntoa, network -
     Internet address manipulation routines

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

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

     unsigned long
     inet_addr(const char *cp);

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

     unsigned long
     inet_lnaof(struct in_addr in);

     struct in_addr
     inet_makeaddr(unsigned long net, unsigned long lna);

     unsigned long
     inet_netof(struct in_addr in);

     unsigned long
     inet_network(const char *cp);

     char *
     inet_ntoa(struct in_addr in);

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

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

DESCRIPTION    [Toc]    [Back]

     The routines inet_aton(), inet_addr() and inet_network() interpret character
 strings representing numbers expressed in the Internet standard
     "dotted quad" notation.

     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, or 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 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
     is invalid.

     The inet_addr() and inet_network() functions return numbers suitable for
     use as Internet addresses and Internet network numbers, respectively.

     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 "dotted quad" notation.

     The routine inet_makeaddr() takes an Internet network number and a local
     network address (both in host order) and constructs an Internet address
     from it.  Note that to convert only a single value to a struct in_addr
     form that value shoud be passed as the first parameter and `0L' should be
     given for the second parameter.

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

     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 the "dotted quad" notation take one of the following


     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 (e.g.  Intel i386, i486
     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 right-most two bytes of the network
     address.  This makes the three part address format convenient for specifying
 Class B network addresses as ``128.net.host''.

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

     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 "dotted quad" notation may be decimal,
 octal, or hexadecimal, as specified in the C language (i.e., a
     leading 0x or 0X implies hexadecimal; otherwise, a leading 0 implies
     octal; otherwise, the number is interpreted as decimal).


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

     The presentation format of an IPv6 address is given in RFC 2373:

     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.  Examples:


          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:


DIAGNOSTICS    [Toc]    [Back]

     The constant INADDR_NONE is returned by inet_addr() and inet_network()
     for malformed requests.

SEE ALSO    [Toc]    [Back]

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

     IP Version 6 Addressing Architecture, RFC, 2373, July 1998.

STANDARDS    [Toc]    [Back]

     The inet_ntop and inet_pton functions conform to .  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

HISTORY    [Toc]    [Back]

     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 and thence NetBSD 1.3.

BUGS    [Toc]    [Back]

     The value INADDR_NONE (0xffffffff) is a valid broadcast address, but
     inet_addr() cannot return that value without indicating failure.  The
     newer inet_aton() function does not share this problem.

     The problem of host byte ordering versus network byte ordering is confusing.

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

     inet_addr() should return a struct in_addr.

BSD                              June 18, 1997                             BSD
[ Back ]
 Similar pages
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inet IRIX Internet address manipulation
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inet_net_ntop FreeBSD Internet network number manipulation routines
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