1.in_addr，在inlcude/netinet.h中声明，用于存储ip地址，声明如下
2.广播、环回、组播等地址，在include/netinet.h中声明，声明如下
typedef uint32_t in_addr_t;
struct in_addr
{
in_addr_t s_addr;
};
2.广播、环回、组播等地址，在include/netinet.h中声明，声明如下
/* Address to accept any incoming messages. */
#define INADDR_ANY ((in_addr_t) 0x00000000)
/* Address to send to all hosts. */
#define INADDR_BROADCAST ((in_addr_t) 0xffffffff)
/* Address indicating an error return. */
#define INADDR_NONE ((in_addr_t) 0xffffffff)
/* Network number for local host loopback. */
#define IN_LOOPBACKNET 127
/* Address to loopback in software to local host. */
#ifndef INADDR_LOOPBACK
# define INADDR_LOOPBACK ((in_addr_t) 0x7f000001) /* Inet 127.0.0.1. */
#endif
/* Defines for Multicast INADDR. */
#define INADDR_UNSPEC_GROUP ((in_addr_t) 0xe0000000) /* 224.0.0.0 */
#define INADDR_ALLHOSTS_GROUP ((in_addr_t) 0xe0000001) /* 224.0.0.1 */
#define INADDR_ALLRTRS_GROUP ((in_addr_t) 0xe0000002) /* 224.0.0.2 */
#define INADDR_MAX_LOCAL_GROUP ((in_addr_t) 0xe00000ff) /* 224.0.0.255 */