inet_addr - Man Page

IPv4 address manipulation


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <arpa/inet.h>

in_addr_t inet_addr(const char *cp);
char *inet_ntoa(struct in_addr in);


The inet_addr() function shall convert the string pointed to by cp, in the standard IPv4 dotted decimal notation, to an integer value suitable for use as an Internet address.

The inet_ntoa() function shall convert the Internet host address specified by in to a string in the Internet standard dot notation.

The inet_ntoa() function need not be thread-safe.

All Internet addresses shall be returned in network order (bytes ordered from left to right).

Values specified using IPv4 dotted decimal notation take one of the following forms:


When four parts are specified, each shall be interpreted as a byte of data and assigned, from left to right, to the four bytes of an Internet address.


When a three-part address is specified, the last part shall be 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 shall be 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 shall be stored directly in the network address without any byte rearrangement.

All numbers supplied as parts in IPv4 dotted decimal notation may be decimal, octal, or hexadecimal, as specified in the ISO C standard (that is, a leading 0x or 0X implies hexadecimal; otherwise, a leading '0' implies octal; otherwise, the number is interpreted as decimal).

Return Value

Upon successful completion, inet_addr() shall return the Internet address. Otherwise, it shall return (in_addr_t)(-1).

The inet_ntoa() function shall return a pointer to the network address in Internet standard dot notation.


No errors are defined.

The following sections are informative.



Application Usage

The return value of inet_ntoa() may point to static data that may be overwritten by subsequent calls to inet_ntoa().



Future Directions


See Also

endhostent(), endnetent()

The Base Definitions volume of POSIX.1-2017, <arpa_inet.h>

Referenced By

arpa_inet.h(0p), endservent(3p).

2017 IEEE/The Open Group POSIX Programmer's Manual