convert Internet addresses between
presentation and network formats
const char *
af, const void * restrict
src, char * restrict
af, const char * restrict
src, void * restrict
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 to presentation format. 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.
All Internet addresses are returned in network order (bytes ordered from left to right).
INTERNET ADDRESSES (IP VERSION 4)
Values must be specified using the standard dot notation:
All four parts must be decimal numbers between 0 and 255,
inclusive, and are 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.
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:
gethostbyname(3), inet_addr(3), inet_net(3), hosts(5)
inet_pton functions conform to the IETF IPv6 BSD API
and address formatting specifications, as well as IEEE Std
inet_ntop functions appeared in BIND 4.9.4.
inet_pton does not accept 1-,
2-, or 3-part dotted addresses; all four parts must be specified and must be
in decimal (and not octal or hexadecimal). 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.