Host Address Data Type - The GNU C Library

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##### 16.6.2.2 Host Address Data Type

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.