INET(3) BSD Programmer's Manual INET(3)
inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr, inet_lnaof,
inet_netof - Internet address manipulation routines
inet_aton(char *cp, struct in_addr *pin);
inet_ntoa(struct in_addr in);
inet_makeaddr(int net, int lna);
inet_lnaof(struct in_addr in);
inet_netof(struct in_addr in);
The routines inet_aton(), inet_addr() and inet_network() interpret char-
acter strings representing numbers expressed in the Internet standard `.'
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 is invalid. The inet_addr() and inet_network() functions
return numbers suitable for use as Internet addresses and Internet net-
work numbers, respectively. The routine inet_ntoa() takes an Internet
address and returns an ASCII string representing the address in `.' nota-
tion. The routine inet_makeaddr() takes an Internet network number and a
local network address and constructs an Internet address from it. The
routines inet_netof() and inet_lnaof() break apart Internet host address-
es, returning the network number and local network address part, respec-
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.
Values specified using the `.' notation take one of the following forms:
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 the VAX the bytes referred to above appear as ``d.c.b.a''. That is,
VAX 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 right-most two bytes of the network ad-
dress. This makes the three part address format convenient for specify-
ing 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 right most three bytes of the network
address. This makes the two part address format convenient for specify-
ing 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 `.' notation may be decimal, oc-
tal, or hexadecimal, as specified in the C language (i.e., a leading 0x
or 0X implies hexadecimal; otherwise, a leading 0 implies octal; other-
wise, the number is interpreted as decimal).
The constant INADDR_NONE is returned by inet_addr() and inet_network()
for malformed requests.
gethostbyname(3), getnetent(3), hosts(5), networks(5),
These functions appeared in 4.2BSD.
The value INADDR_NONE (0xffffffff) is a valid broadcast address, but
inet_addr() cannot return that value without indicating failure. The
newer inet_aton() function does not share this problem. The problem of
host byte ordering versus network byte ordering is confusing. The string
returned by inet_ntoa() resides in a static memory area.
Inet_addr should return a struct in_addr.
4.2 Berkeley Distribution June 4, 1993 2