Java – How to check if an IP address is private or not

This post shows you how to check if an IP address is private or not in Java.

Let’s consider the following code:

package com.bytenota;

public class IPAddress {

    public static boolean isPrivateIPv4(String ipAddress) {
        try {
            String[] ipAddressArray = ipAddress.split("\\.");
            int[] ipParts = new int[ipAddressArray.length];
            for (int i = 0; i < ipAddressArray.length; i++) {
                ipParts[i] = Integer.parseInt(ipAddressArray[i].trim());
            }
            
            switch (ipParts[0]) {
                case 10:
                case 127:
                    return true;
                case 172:
                    return (ipParts[1] >= 16) && (ipParts[1] < 32);
                case 192:
                    return (ipParts[1] == 168);
                case 169:
                    return (ipParts[1] == 254);
            }
        } catch (Exception ex) {
        }
        
        return false;
    }

    public static boolean isPrivateIPv6(String ipAddress) {
        boolean isPrivateIPv6 = false;
        String[] ipParts = ipAddress.trim().split(":");
        if (ipParts.length > 0) {
            String firstBlock = ipParts[0];
            String prefix = firstBlock.substring(0, 2);
            
            if (firstBlock.equalsIgnoreCase("fe80")
                    || firstBlock.equalsIgnoreCase("100")
                    || ((prefix.equalsIgnoreCase("fc") && firstBlock.length() >= 4))
                    || ((prefix.equalsIgnoreCase("fd") && firstBlock.length() >= 4))) {
                isPrivateIPv6 = true;
            }
        }
        return isPrivateIPv6;
    }
}

In the above IPAddress class, we have implemented the following methods:

• isPrivateIPv4: method that checks if an IPv4 address is private or not
• isPrivateIPv6: method that checks if an IPv6 address is private or not

Use the methods:

package com.bytenota;

public class Program {
    
    public static void main(String[] args) {
        // test IPv4
        String ipv4Address = "127.56.87.4";

        if (IPAddress.isPrivateIPv4(ipv4Address)) {
            System.out.println("This is a private IPv4");
        }
        
        
        // test IPv6 
        String ipv6Address = "fe80:db8:a0b:12f0::1";

        if (IPAddress.isPrivateIPv6(ipv6Address)) {
            System.out.println("This is a private IPv6");
        }
    }
}

 
Output:

This is a private IPv4
This is a private IPv6