Calculate broadcast address from ip and subnet mask - c

Question

I want to calculate the broadcast address for:
IP: 192.168.3.1
Subnet: 255.255.255.0
= 192.168.3.255
in C.
I know the way (doing fancy bitwise OR's between the inversed IP and subnet), but my problem is I come from the green fields of MacOSX Cocoa programing.
I looked into the source of ipcal, but wasn't able to integrate it into my code base. There must be a simple ten lines of code somewhere on the internet, I just can't find it.
Could someone point me to a short code example of how to do it in C?

Answer 1

Just calculate:
broadcast = ip | ( ~ subnet )
(Broadcast = ip-addr or the inverted subnet-mask)
The broadcast address has a 1 bit where the subnet mask has a 0 bit.

Answer 2

I understand that the OP had at least a vague understanding of the bit-level arithmetic but was lost on converting the strings to numbers and its inverse. here's a working (with minimal testing anyway) example, using froh42's calculation.
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

Answer 3

Could it be?
unsigned broadcast(unsigned ip,unsigned subnet){
unsigned int bits = subnet ^ 0xffffffff;
unsigned int bcast = ip | bits;
return bcast;
}
Edit: I considered that both ip and subnet are without "."

Answer 4

Here is how to do it in C#. for example using ip 10.28.40.149 with netmask 255.255.252.0 returns 10.28.43.255 which is the correct broadcast address. thanks to some code from here
private static string GetBroadcastAddress(string ipAddress, string subnetMask) {
//determines a broadcast address from an ip and subnet
var ip = IPAddress.Parse(ipAddress);
var mask = IPAddress.Parse(subnetMask);
byte[] ipAdressBytes = ip.GetAddressBytes();
byte[] subnetMaskBytes = mask.GetAddressBytes();
if (ipAdressBytes.Length != subnetMaskBytes.Length)
throw new ArgumentException("Lengths of IP address and subnet mask do not match.");
byte[] broadcastAddress = new byte[ipAdressBytes.Length];
for (int i = 0; i < broadcastAddress.Length; i++) {
broadcastAddress[i] = (byte)(ipAdressBytes[i] | (subnetMaskBytes[i] ^ 255));
}
return new IPAddress(broadcastAddress).ToString();
}

Answer 5

ok whom will look for this code in the future. I have spend sometimes today as I needed this, here is the full code and it works :) simply copy and paste it and then import the required dlls.
private IPAddress CalculateBroadCastAddress(IPAddress currentIP, IPAddress ipNetMask)
{
string[] strCurrentIP = currentIP.ToString().Split('.');
string[] strIPNetMask = ipNetMask.ToString().Split('.');
ArrayList arBroadCast = new ArrayList();
for (int i = 0; i < 4; i++)
{
int nrBCOct = int.Parse(strCurrentIP[i]) | (int.Parse(strIPNetMask[i]) ^ 255);
arBroadCast.Add(nrBCOct.ToString());
}
return IPAddress.Parse(arBroadCast[0] + "." + arBroadCast[1] +
"." + arBroadCast[2] + "." + arBroadCast[3]);
}
private IPAddress getIP()
{
IPHostEntry host = Dns.GetHostEntry(Dns.GetHostName());
foreach (IPAddress ip in host.AddressList)
{
if (ip.AddressFamily == AddressFamily.InterNetwork)
{
return ip;
}
}
return null;
}
private IPAddress getSubnetMask()
{
NetworkInterface[] Interfaces = NetworkInterface.GetAllNetworkInterfaces();
IPAddress ip = getIP();
foreach (NetworkInterface interf in Interfaces)
{
UnicastIPAddressInformationCollection UnicastIPInfoCol = interf.GetIPProperties().UnicastAddresses;
foreach (UnicastIPAddressInformation UnicatIPInfo in UnicastIPInfoCol)
{
if (UnicatIPInfo.Address.Equals(ip))
return UnicatIPInfo.IPv4Mask;
}
}
return null;
}
Then just call it like :
IPAddress broadcastip = CalculateBroadCastAddress(getIP(), getSubnetMask());
Happy coding :)