C实现判断两个IP是否在同一个子网

在进行网络程序的时候避免不了对给定IP是否跨子网段进行判断。相关原理倒是简单， 贴出相关代码:

 1 #include <stdio.h>

 2 #include <math.h>

 3 #include <string.h>

 4 #include <stdlib.h>

 5 #include <unistd.h>

 6  

 7 #define ALL_BIT 32 /* ip address have 32 bits */

 8  

 9 /*

10 ** description: juge the ip_a and ip_b is in the same subnet or not

11 ** this functio is used IPv4 only, and did not check the input parameter's

12 ** correctness, so make sure the value of parameter before call it

13 ** para: sub_mask is a decimal subnet mask , not dotted decimal

14 ** ip_a/ip_b: the ip address string, in dotted decimal

15 ** return: 0 for the same, -1 for the not

16 */

17 int same_subnet (int sub_mask, char *ip_a, char *ip_b)

18 {

19   int mask = 0xFFFFFFFF;

20   mask = mask << (ALL_BIT - sub_mask);

21  

22   double cnt_a = 0.0;

23   double cnt_b = 0.0;

24   double tmp = 0.0;

25  

26   char *token = NULL;

27  

28  int i = 3;

29  

30   for (token = strtok (ip_a, "."); token != NULL; token = strtok (NULL, ".")) {

31     tmp = atoi (token);

32     tmp = tmp * pow (256, i--);  /* what dose this mean? do you understand */

33     cnt_a += tmp;

34   }

35  

36   i = 3; /* reset i */

37   for (token = strtok (ip_b, "."); token != NULL; token = strtok (NULL, ".")) {

38     tmp = atoi (token);

39     tmp = tmp * pow (256, i--);

40     cnt_b += tmp;

41   }

42  

43   unsigned long mask_a = (unsigned long)cnt_a;

44   unsigned long mask_b = (unsigned long)cnt_b;

45  

46   //printf ("mask_a %u\tmask_b %u\n", mask_a, mask_b);

47  

48   mask_a &= mask;   /* get the ip's netmask for compare */

49   mask_b &= mask;

50   //printf ("mask_a %u\tmask_b %u\n", mask_a, mask_b);

51  

52   if (mask_a == mask_b)

53   return 0;

54   else

55     return -1;

56 }

57  

58 /*

59 ** test code

60 */

61 int main (int argc, char **argv)

62 {

63   int i = same_subnet (atoi (argv[1]), argv[2], argv[3]);

64   printf ("%s subnet\n", i == 0? "same":"not same");

65   return 0;

66 }

67  

68  

69  

70 代码在gcc version 4.7.2 20121109 (Red Hat 4.7.2-8) (GCC)编译通过。