INET_NTOP(3) Library Functions Manual INET_NTOP(3)
inet_ntop, inet_pton - convert Internet addresses between presentation
and network formats
const char *
inet_ntop(int af, const void * restrict src, char * restrict dst,
inet_pton(int af, const char * restrict src, void * restrict dst);
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 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
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
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:
gethostbyname(3), inet_addr(3), inet_net(3), hosts(5)
The inet_ntop and inet_pton functions conform to the IETF IPv6 BSD API
and address formatting specifications, as well as IEEE Std 1003.1-2008
The inet_pton and inet_ntop functions appeared in BIND 4.9.4.
Note that 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,
OpenBSD 5.7 May 9, 2014 OpenBSD 5.7
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