从ip和子网掩码计算广播地址

Question

我想计算广播地址:

IP:     192.168.3.1
Subnet: 255.255.255.0
=       192.168.3.255

在C.

我知道的方式(在反向的IP和子网之间做出花哨的按位OR),但我的问题是我来自MacOSX Cocoa编程的绿色领域.

我查看了ipcal的来源,但无法将其集成到我的代码库中.互联网上必须有一行简单的十行代码,我找不到它.有人能指出我在C中如何做到的简短代码示例吗？

Answer 1

算一算:

broadcast = ip | ( ~ subnet )

(Broadcast = ip-addr或反向子网掩码)

广播地址有一个1子网掩码有0位的位.

Answer 2

我理解OP至少对位级算术有一个模糊的理解,但在将字符串转换为数字及其反转时丢失了.这是一个工作(无论如何最小的测试)的例子,使用froh42的计算.

jcomeau@aspire:~/rentacoder/jcomeau/freifunk$ cat inet.c; make inet; ./inet 192.168.3.1 255.255.255.0
#include <arpa/inet.h>
#include <stdio.h>
int main(int argc, char **argv) {
    char *host_ip = argc > 1 ? argv[1] : "127.0.0.1";
    char *netmask = argc > 2 ? argv[2] : "255.255.255.255";
    struct in_addr host, mask, broadcast;
    char broadcast_address[INET_ADDRSTRLEN];
    if (inet_pton(AF_INET, host_ip, &host) == 1 &&
        inet_pton(AF_INET, netmask, &mask) == 1)
        broadcast.s_addr = host.s_addr | ~mask.s_addr;
    else {
        fprintf(stderr, "Failed converting strings to numbers\n");
        return 1;
    }
    if (inet_ntop(AF_INET, &broadcast, broadcast_address, INET_ADDRSTRLEN) != NULL)
        printf("Broadcast address of %s with netmask %s is %s\n",
            host_ip, netmask, broadcast_address);
    else {
        fprintf(stderr, "Failed converting number to string\n");
        return 1;
    }
    return 0;
}
cc     inet.c   -o inet
Broadcast address of 192.168.3.1 with netmask 255.255.255.0 is 192.168.3.255