C语言中判断两个IPv4地址是否属于同一个子网的代码

int _to_int(char * str, int start_idx, int end_idx)
{
 int a = 0, i;
 for (i = start_idx; i <= end_idx; ++i)
 {
 a = a * 10 + (str[i] - '0');
 }

 return a;
}

/*
 * 将ip字符串转化为4字节的整形
 */
int ip_to_int(char * ip)
{
 int start = 0, i = 0, ret = 0;
 int shift_factor = 3; // 一开始要向右移动3 * 8位
 char c;
 while (c = ip[i])
 {
 if (c == '.')
 {
  int a = _to_int(ip, start, i - 1);
  int temp = shift_factor * 8;
  ret = ret | (a << temp);

  shift_factor--;
  start = i + 1;
 }
 i++;
 }

 return ret;
}

#include <stdio.h>

int main()
{
 char a1[15], a2[15], a3[15];

 gets(a1);
 gets(a2);
 gets(a3);

 int ip1 = ip_to_int(a1);
 int ip2 = ip_to_int(a2);
 int ip3 = ip_to_int(a3);

 int result = 0;
 if ((ip1 & ip3) == (ip2 & ip3))
 {
 result = 1;
 }

 printf("%d", result);

 return 0;
}

#include<iostream>
#include<fstream>
#include<string>
#include<cstring>
#include<cctype>
#include<algorithm>
#include<cstdlib>
using namespace std;

typedef struct ip
{
 int first;
 int second;
 int three;
} IP;

int judgeIp(string ipSubNet,IP &ip)
{
 int index=0;
 ip.first=atoi(&ipSubNet[index]);
 if(ip.first>255)
 return 0;

 index++;
 index=ipSubNet.find_first_of('.',index);
 ip.second=atoi(&ipSubNet[++index]);
 if(ip.second>255)
 return 0;

 index++;
 index=ipSubNet.find_first_of('.',index);
 ip.three=atoi(&ipSubNet[++index]);
 if(ip.three>255)
 return 0;

 index++;
 index=ipSubNet.find_first_of('.',index);
 ip.fouth=atoi(&ipSubNet[++index]);
 if(ip.fouth>255)
 return 0;

 return 1;
}

int main()
{
 string ipSubNet,ipAdd1,ipAdd2;
 IP subNet,ip1,ip2;
 while(cin>>ipSubNet>>ipAdd1>>ipAdd2)
 {
 if(judgeIp(ipSubNet,subNet)&&judgeIp(ipAdd1,ip1)&&judgeIp(ipAdd2,ip2))
 {
  ip1.first=ip1.first & subNet.first;
  ip1.second=ip1.first & subNet.second;
  ip1.second=ip1.first & subNet.second;
  ip1.fouth=ip1.first & subNet.fouth;

  ip2.first=ip2.first & subNet.first;
  ip2.second=ip2.first & subNet.second;
  ip2.second=ip2.first & subNet.second;
  ip2.fouth=ip2.first & subNet.fouth;

  if(ip1.first==ip2.first&&ip1.second==ip2.second&&ip1.three==ip2.three&&ip1.fouth==ip2.fouth)
  cout<<'0'<<endl;
  else
  cout<<'2'<<endl;
 }
 else
  cout<<'1'<<endl;
 }
 return 0;
}

ip_addr.h

#define ip_addr_netcmp(addr1, addr2, mask) (((addr1)->addr & \
                       (mask)->addr) == \
                       ((addr2)->addr & \
                       (mask)->addr))