cc [ flag ... ] file ... -lsocket -lnsl [ library ... ] #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h>const char *inet_ntop(int af, const void *addr, char *cp, size_t size);
The inet_ntop() and inet_pton() routines can manipulate both IPv4 and IPv6 addresses, whereas inet_addr(), inet_network(), inet_makeaddr(), inet_lnaof(), inet_netof(), and inet_ntoa() can only manipulate IPv4 addresses.
The inet_ntop() routine converts a numeric address into a string suitable for presentation. The af argument specifies the family of the address. This can be AF_INET or AF_INET6. The addr argument points to a buffer holding an IPv4 address if the af argument is AF_INET, or an IPv6 address if the af argument is AF_INET6; the address must be in network byte order. The cp argument points to a buffer where the routine will store the resulting string. The size argument specifies the size of this buffer. The application must specify a non-NULL cp argument. For IPv6 addresses, the buffer must be at least 46-octets. For IPv4 addresses, the buffer must be at least 16-octets. In order to allow applications to easily declare buffers of the proper size to store IPv4 and IPv6 addresses in string form, the following two constants are defined in <netinet/in.h>:
#define INET_ADDRSTRLEN 16 #define INET6_ADDRSTRLEN 46
The inet_addr() and inet_network() routines interpret character strings representing numbers expressed in the IPv4 standard `.' notation, returning numbers suitable for use as IPv4 addresses and IPv4 network numbers, respectively. The routine inet_makeaddr() takes an IPv4 network number and a local network address and constructs an IPv4 address from it. The routines inet_netof() and inet_lnaof() break apart IPv4 host addresses, returning the network number and local network address part, respectively.
The inet_ntoa() routine returns a pointer to a string in the base 256 notation d.d.d.d. See INTERNET ADDRESSES.
Internet addresses are returned in network order, bytes ordered from left to right. Network numbers and local address parts are returned as machine format integer values.
Note that it is not necessary to write the leading zeros in an individual field. However, there must be at least one numeral in every field, except as described below.
Due to some methods 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,
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 standard IPv4 representation address, for example,
::FFFF:220.127.116.11 ::18.104.22.168where “::FFFF:d.d.d.d” and “::d.d.d.d” are, respectively, the general forms of an IPv4–mapped IPv6 address and an IPv4–compatible IPv6 address. Note that the IPv4 portion must be in the “d.d.d.d” form. The following forms are invalid:
::FFFF:d.d.d ::FFFF:d.d ::d.d.d ::d.dThe following form:
::FFFF:dis valid, however it is an unconventional representation of the IPv4–compatible IPv6 address,
::255.255.0.dwhile “::d” corresponds to the general IPv6 address “0:0:0:0:0:0:0:d”.
d.d.d.d d.d.d d.d d
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.
With the exception of inet_pton(), numbers supplied as parts in `.' notation may be decimal, octal, or hexadecimal, as specified in the C language. For example, a leading 0x or 0X implies hexadecimal; otherwise, a leading 0 implies octal; otherwise, the number is interpreted as decimal.
The inet_ntop() routine returns a pointer to the buffer containing a string if the conversion succeeds, and NULL otherwise. Upon failure, errno is set to EAFNOSUPPORT if the af argument is invalid or ENOSPC if the size of the result buffer is inadequate.
inet_pton() returns 1 if the conversion succeeds, 0 if the input is not a valid IPv4 dotted-decimal string or a valid IPv6 address string, or –1 with errno set to EAFNOSUPPORT if the af argument is unknown.
The value -1 is returned by inet_addr() and inet_network() for malformed requests.
The routines inet_netof() and inet_lnaof() break apart IPv4 host addresses, returning the network number and local network address part, respectively.
The routine inet_ntoa() returns a pointer to a string in the base 256 notation d.d.d.d described in INTERNET ADDRESSES.
See attributes(5) for descriptions of the following attributes:
|ATTRIBUTE TYPE||ATTRIBUTE VALUE|
The return value from inet_ntoa() points to a buffer which is overwritten on each call. This buffer is implemented as thread-specific data in multithreaded applications.
The problem of host byte ordering versus network byte ordering is confusing. A simple way to specify Class C network addresses in a manner similar to that for Class B and Class A is needed.