IP地址結構：

   struct   in_addr {

                     union {

                            struct {

                                        unsigned  char s_b1,

                                                                s_b2,

                                                                s_b3,

                                                                s_b4;

                            } S_un_b；

                            struct {

                                        unsigned short s_w1,

                                                                 s_w2;

                            } S_un_w;

                            unsigend long S_addr;

                      }S_un;

                };

以上所指出的IP地址為IPV4,長度為32bit.unsigned char 為8bit,unsigned short 為16bit,unsigned long 為32bit.如果要表示一個IP地址：10.14.25.90依據上面的結構體，可以有3種不同的方式：假設定義結構體 in_addr sin_addr;

1.sin_addr.S_un.S_un_b.s_b1 = 10;  

       sin_addr.S_un.S_un_b.s_b1 = 14;

       sin_addr.S_un.S_un_b.s_b1 = 25;

       sin_addr.S_un.S_un_b.s_b1 = 90;

2.sin_addr.S_un.Sun_w.s_w1 = (14<<8)|10;

        sin_addr.S_un.Sun_w.s_w2 = (90<<8)|25;

3.sin_addr.S_un.S_addr = (90<<24)|(25<<16)|(14<<8)|10;