— Internet address manipulation
char *cp, struct in_addr
af, const char
const char *
af, const void
*src, char *dst,
inet_network() interpret character strings
representing numbers expressed in the Internet standard “dot”
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.
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
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
converts an address from network format (usually a
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.
takes an Internet address and returns an ASCII string representing the
address in dot notation.
takes an Internet network number and a local network address and constructs
an Internet address from it.
break apart Internet host addresses, returning the network number and local
network address part, respectively.
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.
INTERNET ADDRESSES (IP VERSION 4)
Values specified using dot notation take one of the following forms:
a.b.c.d a.b.c a.b a
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 left.
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).
INTERNET ADDRESSES (IP VERSION 6)
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:
- 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.).
- 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
- 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), networks(5)
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
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.
functions appeared in 4.2BSD. The
functions appeared in 4.3BSD. The
functions appeared in BIND 4.9.4.
INADDR_NONE (0xffffffff) is a
valid broadcast address, but
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
The problem of host byte ordering versus network byte ordering is confusing.
The string returned by
in a static memory area.