Section: Linux Programmer's Manual (3)
Updated: 2001-07-25
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inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr, inet_lnaof, inet_netof - Internet address manipulation routines  


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

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

in_addr_t inet_addr(const char *cp);

in_addr_t inet_network(const char *cp);

char *inet_ntoa(struct in_addr in);

struct in_addr inet_makeaddr(int net, int host);

in_addr_t inet_lnaof(struct in_addr in);

in_addr_t inet_netof(struct in_addr in);


inet_aton() converts the Internet host address cp from the standard numbers-and-dots notation into binary data and stores it in the structure that inp points to. inet_aton() returns non-zero if the address is valid, zero if not.

The inet_addr() function converts the Internet host address cp from numbers-and-dots notation into binary data in network byte order. If the input is invalid, INADDR_NONE (usually -1) is returned. This is an obsolete interface to inet_aton(), described immediately above; it is obsolete because -1 is a valid address (, and inet_aton() provides a cleaner way to indicate error return.

The inet_network() function extracts a number suitable for use as an Internet address in host byte order from the address cp in numbers-and-dots notation. If the input is invalid, -1 is returned.

The inet_ntoa() function converts the Internet host address in given in network byte order to a string in standard numbers-and-dots notation. The string is returned in a statically allocated buffer, which subsequent calls will overwrite.

The inet_makeaddr() function makes an Internet host address in network byte order by combining the network number net with the local address host in network net, both in local host byte order.

The inet_lnaof() function returns the local host address part of the Internet address in. The local host address is returned in local host byte order.

The inet_netof() function returns the network number part of the Internet Address in. The network number is returned in local host byte order.

The structure in_addr as used in inet_ntoa(), inet_makeaddr(), inet_lnoaf() and inet_netof() is defined in netinet/in.h as:

struct in_addr {
        unsigned long int s_addr;

Note that on the i80x86 the host byte order is Least Significant Byte first (little endian), whereas the network byte order, as used on the Internet, is Most Significant Byte first (big endian).  


When you using numbers-and-dots notation for addresses, be aware that each number will be interpreted as octal if preceded by a 0 and as hexadecimal if preceded by 0x. For example, inet_aton("", &t) will interpret the address as and not  


In order to expose the declaration of inet_aton(), one of the feature test macros _BSD_SOURCE, _SVID_SOURCE, or _GNU_SOURCE must be defined.  


4.3BSD. inet_addr(), inet_aton(), and inet_ntoa() are specified in POSIX.1-2001.  


gethostbyname(3), getnetent(3), inet_ntop(3), inet_pton(3), hosts(5), networks(5)



SEE ALSO manual pages