IPv4 Internet host addresses are represented in some contexts as integers
(type uint32_t
). In other contexts, the integer is
packaged inside a structure of type struct in_addr
. It would
be better if the usage were made consistent, but it is not hard to extract
the integer from the structure or put the integer into a structure.
You will find older code that uses unsigned long int
for
IPv4 Internet host addresses instead of uint32_t
or struct
in_addr
. Historically unsigned long int
was a 32-bit number but
with 64-bit machines this has changed. Using unsigned long int
might break the code if it is used on machines where this type doesn’t
have 32 bits. uint32_t
is specified by Unix98 and guaranteed to have
32 bits.
IPv6 Internet host addresses have 128 bits and are packaged inside a
structure of type struct in6_addr
.
The following basic definitions for Internet addresses are declared in the header file netinet/in.h:
This data type is used in certain contexts to contain an IPv4 Internet
host address. It has just one field, named s_addr
, which records
the host address number as an uint32_t
.
uint32_t
INADDR_LOOPBACK ¶You can use this constant to stand for “the address of this machine,”
instead of finding its actual address. It is the IPv4 Internet address
‘127.0.0.1’, which is usually called ‘localhost’. This
special constant saves you the trouble of looking up the address of your
own machine. Also, the system usually implements INADDR_LOOPBACK
specially, avoiding any network traffic for the case of one machine
talking to itself.
uint32_t
INADDR_ANY ¶You can use this constant to stand for “any incoming address” when
binding to an address. See Setting the Address of a Socket. This is the usual
address to give in the sin_addr
member of struct sockaddr_in
when you want to accept Internet connections.
uint32_t
INADDR_BROADCAST ¶This constant is the address you use to send a broadcast message.
uint32_t
INADDR_NONE ¶This constant is returned by some functions to indicate an error.
This data type is used to store an IPv6 address. It stores 128 bits of data, which can be accessed (via a union) in a variety of ways.
struct in6_addr
in6addr_loopback ¶This constant is the IPv6 address ‘::1’, the loopback address. See
above for a description of what this means. The macro
IN6ADDR_LOOPBACK_INIT
is provided to allow you to initialize your
own variables to this value.
struct in6_addr
in6addr_any ¶This constant is the IPv6 address ‘::’, the unspecified address. See
above for a description of what this means. The macro
IN6ADDR_ANY_INIT
is provided to allow you to initialize your
own variables to this value.