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INET(3) Library Functions Manual INET(3)

inet_addr, inet_aton, inet_lnaof, inet_makeaddr, inet_netof, inet_network, inet_ntoa, inet_ntop, inet_pton
Internet address manipulation routines

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

inet_addr(const char *cp);


inet_lnaof(struct in_addr in);

struct in_addr
inet_makeaddr(in_addr_t net, in_addr_t lna);

inet_netof(struct in_addr in);

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);



The routines inet_aton(), inet_addr(), and inet_network() interpret character strings representing numbers expressed in the Internet standard “dot” 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; 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 AF_INET6. 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 was 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 dot notation. The routine inet_makeaddr() takes an Internet network number and a local network address and constructs an Internet address from it. The routines inet_netof() and inet_lnaof() 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.

Values specified using dot notation take one of the following forms:

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 “”.


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).

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


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:


    may be represented as:

    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:




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


The inet_ntop and 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 inet_aton.

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.

The value INADDR_NONE (0xffffffff) is a valid broadcast address, but inet_addr() cannot 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 inet_aton() instead.



December 9, 2008 OpenBSD-5.1