How to get subnet mask of local system using java?

Question

How to get Subnet mask address of local system using Java programming?

Answer 1

I devised an IPv4 only solution that is simple enough. I needed that to generate netmask for subnetworks here in order to delegate those subnets correctly. I know I could have generated a table of the 32 possible masks, but I prefered to get it computed each time.

So here is my solution.

/*
 * Get network mask for the IP address and network prefix specified...
 * The network mask will be returned has an IP, thus you can
 * print it out with .getHostAddress()...
 */
public static InetAddress getIPv4LocalNetMask(InetAddress ip, int netPrefix) {

    try {
        // Since this is for IPv4, it's 32 bits, so set the sign value of
        // the int to "negative"...
        int shiftby = (1<<31);
        // For the number of bits of the prefix -1 (we already set the sign bit)
        for (int i=netPrefix-1; i>0; i--) {
            // Shift the sign right... Java makes the sign bit sticky on a shift...
            // So no need to "set it back up"...
            shiftby = (shiftby >> 1);
        }
        // Transform the resulting value in xxx.xxx.xxx.xxx format, like if
        /// it was a standard address...
        String maskString = Integer.toString((shiftby >> 24) & 255) + "." + Integer.toString((shiftby >> 16) & 255) + "." + Integer.toString((shiftby >> 8) & 255) + "." + Integer.toString(shiftby & 255);
        // Return the address thus created...
        return InetAddress.getByName(maskString);
    }
        catch(Exception e){e.printStackTrace();
    }
    // Something went wrong here...
    return null;
}

You just call it with the IP and the prefix you want to use, it will generate the netmask for you.

Answer 2

In summary, a method to obtain the mask would be like this:

public String mascara() throws SocketException{
try{
InetAddress localHost = Inet4Address.getLocalHost();
NetworkInterface networkInterface = NetworkInterface.getByInetAddress(localHost);
prefijo = 
""+networkInterface.getInterfaceAddresses().get(0).getNetworkPrefixLength();
int shft = 0xffffffff<<(32- 
networkInterface.getInterfaceAddresses().get(0).getNetworkPrefixLength());
int oct1 = ((byte) ((shft&0xff000000)>>24)) & 0xff;
int oct2 = ((byte) ((shft&0x00ff0000)>>16)) & 0xff;
int oct3 = ((byte) ((shft&0x0000ff00)>>8)) & 0xff;
int oct4 = ((byte) (shft&0x000000ff)) & 0xff;
mascara = oct1+"."+oct2+"."+oct3+"."+oct4;
// System.out.println(""+mascara);           
}catch(UnknownHostException e){
System.out.println("Error: "+e);
}
return mascara;
}

Answer 3

the netmask of the first address of the localhost interface:

InetAddress localHost = Inet4Address.getLocalHost();
NetworkInterface networkInterface = NetworkInterface.getByInetAddress(localHost);
networkInterface.getInterfaceAddresses().get(0).getNetworkPrefixLength();

a more complete approach:

InetAddress localHost = Inet4Address.getLocalHost();
NetworkInterface networkInterface = NetworkInterface.getByInetAddress(localHost);

for (InterfaceAddress address : networkInterface.getInterfaceAddresses()) {
    System.out.println(address.getNetworkPrefixLength());
}

/24 means 255.255.255.

Answer 4

Here is an answer, how to get a submask from WIFI connection: link

I adapted it for my needs, and here it is:

private static String intToIP(int ipAddress) {
    String ret = String.format("%d.%d.%d.%d", (ipAddress & 0xff),
            (ipAddress >> 8 & 0xff), (ipAddress >> 16 & 0xff),
            (ipAddress >> 24 & 0xff));

    return ret;
}

public static String GetSubnetMask_WIFI() {

    WifiManager wifiManager = (WifiManager) Global.getMainActivity()
            .getSystemService(Context.WIFI_SERVICE);
    WifiInfo wifiInfo = wifiManager.getConnectionInfo();

    DhcpInfo dhcp = wifiManager.getDhcpInfo();
    String mask = intToIP(dhcp.netmask);

    return mask;
}