NAME

inet_addr, inet_ntoa — IPv4 address manipulation

SYNOPSIS

[OB] #include <arpa/inet.h>

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

DESCRIPTION

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:

a.b.c.d

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.

a.b.c

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 "128.net.host".

a.b

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 "net.host".

a

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.

ERRORS

No errors are defined.

EXAMPLES

None.

APPLICATION USAGE

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

Applications should prefer inet_pton() over inet_addr() for the following reasons:

• The return value from inet_addr() when converting 255.255.255.255 is indistinguishable from an error.

• The inet_pton() function supports multiple address families.

• The alternative textual representations supported by inet_addr() (but not by inet_pton()) are often used maliciously to confuse or mislead users (e.g., for phishing).

Applications should prefer inet_ntop() over inet_ntoa() as it supports multiple address families and is thread-safe.

RATIONALE

None.

FUTURE DIRECTIONS

These functions are included only for compatibility with older implementations and may be removed in a future version.

SEE ALSO

endhostent, endnetent, inet_ntop

XBD <arpa/inet.h>

CHANGE HISTORY

First released in Issue 6. Derived from the XNS, Issue 5.2 specification.

Issue 7

Austin Group Interpretation 1003.1-2001 #156 is applied.

Issue 8

Austin Group Defects 1101 and 1102 are applied, marking inet_addr() and inet_ntoa() as obsolescent.