How to Determine If an IP Address Belongs to a Specific Network or Netmask in Java

Question

How can I check if a given IP address is part of a specific network or netmask using Java?

import java.net.InetAddress;
import java.net.UnknownHostException;

public class IPNetworkCheck {
    public static boolean isIPInNetwork(String ip, String network, String netmask) throws UnknownHostException {
        InetAddress ipAddress = InetAddress.getByName(ip);
        InetAddress networkAddress = InetAddress.getByName(network);
        InetAddress netmaskAddress = InetAddress.getByName(netmask);

        byte[] ipBytes = ipAddress.getAddress();
        byte[] networkBytes = networkAddress.getAddress();
        byte[] netmaskBytes = netmaskAddress.getAddress();

        for (int i = 0; i < ipBytes.length; i++) {
            if ((ipBytes[i] & netmaskBytes[i]) != (networkBytes[i] & netmaskBytes[i])) {
                return false;
            }
        }
        return true;
    }
}

Answer

This guide will demonstrate how to determine whether a given IP address falls within a specific network or subnet defined by a netmask in Java. Knowing how to perform this check is valuable for network programming, security, and filtering traffic.

import java.net.InetAddress;
import java.net.UnknownHostException;

public class IPNetworkCheck {
    public static boolean isIPInNetwork(String ip, String network, String netmask) throws UnknownHostException {
        InetAddress ipAddress = InetAddress.getByName(ip);
        InetAddress networkAddress = InetAddress.getByName(network);
        InetAddress netmaskAddress = InetAddress.getByName(netmask);

        byte[] ipBytes = ipAddress.getAddress();
        byte[] networkBytes = networkAddress.getAddress();
        byte[] netmaskBytes = netmaskAddress.getAddress();

        for (int i = 0; i < ipBytes.length; i++) {
            if ((ipBytes[i] & netmaskBytes[i]) != (networkBytes[i] & netmaskBytes[i])) {
                return false;
            }
        }
        return true;
    }
}

Causes

Incorrectly formatted IP addresses
Netmask inaccuracies
Not accounting for different IP classes

Solutions

Use the InetAddress class for reliable IP address handling
Verify that the IP addresses and netmask are correctly formatted
Consider using third-party libraries like Apache Commons IP for advanced scenarios

Common Mistakes

Mistake: Using incorrect IP address formats (e.g., missing segments)

Solution: Ensure IP addresses are in valid formats (e.g., 192.168.1.1, 255.255.255.0).

Mistake: Neglecting to validate if input is an IP or netmask

Solution: Add input validation to confirm that the strings represent valid IP addresses or netmasks.

Helpers

Java check IP address network
netmask check Java
Java IP networking
IP address validation Java
Java InetAddress example