CODEWITHSUNDEEP
javasocketsnetwork-programmingipv6ipv4
Legato
Legato

Reputation: 610

Browser shows IPv6, yet query returns IPv4 -- why?

A bit of context: For study/practice, I'm working on a network application and I noticed that the socket only correctly opens if I provide an IPv4 address to people whose public IP is IPv4 and IPv6 for those whose public IP is IPv6.

I thought to select filter the right one by using something kindred to the following:

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.net.URL;
import java.net.MalformedURLException;

public class Test {
    public static void main(String[] args) throws MalformedURLException, IOException { 
        BufferedReader ipTypeChecker = new BufferedReader(
            new InputStreamReader(
                new URL("https://wtfismyip.com/text").openStream()
            )
        );

        System.out.println(ipTypeChecker.readLine());
    }
}

If I simply google What is my ip, go to this page or this page, My IP is shown as an IPv6 address, yet if I query those pages using the above code it returns an IPv4 address, why?

Of course, if I am suffering from the XY problem feel free to point out where I've gone wrong.

Upvotes: 2

Views: 361

Answers (1)

Steffen Ullrich
Steffen Ullrich

Reputation: 123501

According to the documentation Java prefers IPv4 addresses to IPv6 addresses if both are available:

java.net.preferIPv6Addresses (default: false)

If IPv6 is available on the operating system the default preference is to prefer an IPv4-mapped address over an IPv6 address. This is for backward compatibility reasons...

The target host has both types of address:

wtfismyip.com.          3600    IN      AAAA    2604:4300:a:2c::1:1
wtfismyip.com.          3600    IN      A       69.30.217.90

This means Java will pick the IPv4 address. Browsers will instead prefer IPv6 usually.

Upvotes: 4

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