Coredump

While working on a custom packet injector for Linux, I had to calculate the network and broadcast IP addresses given an IP address and a netmask. After searching on Google I found that the calculations are some very simple bitwise operations.

To get the network address perform a bitwise AND on the IP address and the netmask:
network = ip & netmask

To get the broadcast address perform a bitwise OR on the network address and the complement of the netmask:
broadcast = network | ~netmask

Here a simple C program you can use to calculate the network and broadcast addresses on your own:

#include <stdio.h>
#include <stdlib.h>
#include <arpa/inet.h>

int main(int argc, char **argv)
{
    struct in_addr ip;
    struct in_addr netmask;
    struct in_addr network;
    struct in_addr broadcast;

    if (argc != 3)
    {
        printf("usage: %s ip netmask\n", argv[0]);
        exit(0);
    }   

    inet_aton(argv[1], &ip);
    inet_aton(argv[2], &netmask);

    // bitwise AND of ip and netmask gives the network
    network.s_addr = ip.s_addr & netmask.s_addr;
    printf("network %s\n", inet_ntoa(network));

    // bitwise OR of network and complement of netmask gives the broadcast
    broadcast.s_addr = network.s_addr | ~netmask.s_addr;
    printf("broadcast %s\n", inet_ntoa(broadcast));

    return 0;
}

To compile it use the command:

gcc -Wall ipcalc.c -o ipcalc

The usage is really simple:

$ ./ipcalc
usage: ./ipcalc ip netmask
$ ./ipcalc 192.168.1.0 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255

Feel free to use it and send me comments/suggestions