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:
— Data Type: struct in_addr
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.
— Macro: 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.
— Macro: uint32_t INADDR_ANY
You can use this constant to stand for “any incoming address” when
binding to an address. See Setting Address. This is the usual
address to give in the sin_addr member of struct sockaddr_in when you want to accept Internet connections.
— Macro: uint32_t INADDR_BROADCAST
This constant is the address you use to send a broadcast message.
— Macro: uint32_t INADDR_NONE
This constant is returned by some functions to indicate an error.
— Data Type: struct in6_addr
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.
— Constant: 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.
— Constant: 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.
Published under the terms of the GNU General Public License
