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(libc.info)Host Address Data Type

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Host Address Data Type
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   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.  Note: 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.