INET_ADDR(3) | Library Functions Manual | INET_ADDR(3)

# NAME

`inet_aton`, `inet_addr`, `inet_network`, `inet_ntoa` — Internet Protocol version 4 (IPv4) address manipulation routines

# SYNOPSIS

`#include <arpa/inet.h>`

int
`inet_aton`(const char *cp, struct in_addr *addr);

in_addr_t
`inet_addr`(const char *cp);

in_addr_t
`inet_network`(const char *cp);

char *
`inet_ntoa`(struct in_addr in);

# DESCRIPTION

The functions presented here only support IPv4 addresses. In order to support IPv6 addresses as well, inet_ntop(3) and inet_pton(3) should be used rather than the functions presented here. Scoped IPv6 addresses are supported via getaddrinfo(3) and getnameinfo(3).

The routines `inet_aton`(), `inet_addr`(), and `inet_network`() interpret character strings representing numbers expressed in the Internet standard “dot” notation.

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. Both functions return the constant `INADDR_NONE` if the specified character string is malformed.

The routine `inet_ntoa`() takes an Internet address and returns an ASCII string representing the address in dot notation.

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.

# INTERNET ADDRESSES (IP VERSION 4)

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

    a.b.c.d
    a.b.c
    a.b
    a

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 AMD64 or ARM 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 “`128.net.host`”.

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 “`net.host`”.

When only one part is given, the value is stored directly in the network address without any byte rearrangement.

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).

# SEE ALSO

gethostbyname(3), htonl(3), inet_lnaof(3), inet_net_ntop(3), inet_ntop(3), hosts(5)

# STANDARDS

The `inet_addr` and `inet_ntoa` functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

# HISTORY

The `inet_addr` and `inet_network` functions appeared in 4.2BSD. The `inet_aton` and `inet_ntoa` functions appeared in 4.3BSD.

# BUGS

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.

The problem of host byte ordering versus network byte ordering is confusing.

The string returned by `inet_ntoa`() resides in a static memory area.

March 6, 2024 | Debian