inet_ntoa (3p) - Linux Manuals

inet_ntoa: 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.


inet_addr, inet_ntoa - IPv4 address manipulation


#include <arpa/inet.h>

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


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 reentrant. A function that is not required to be reentrant is not required to 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).


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.




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






Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .


endhostent(), endnetent(), the Base Definitions volume of IEEE Std 1003.1-2001, <arpa/inet.h>