Java

The Javadoc for the appropriate class is Class InetAddress

IP types

The class java.net.InetAddress has two subclasses

• java.net.Inet4Address
• java.net.Inet6Address

These cannot be instantiated directly. An InetAddress can be contructed from the bytes of the address: 4 bytes for an IPv4 address, and 16 bytes for an IPv6 address. The method getByName() can take a string representation ("127.0.0.1" or "::1" for example) and create an InetAddress.

You can tell which type of InetAddress you have using instanceof.

There are methods in each class to tell what type of address you have. For IPv4, these are isLinkLocalAddress(), isMulticastAddress() and others. For IPv6 these are isLinkLocalAddress(), isMulticastAddress() and others. There is no isUniqueLocal() but there is IsSiteLocalAddress() which shouldn't really be there.

Get one DNS address

Java has the class InetAddress in package java.net. This class has the static method getByName() which will create an InetAddress object from a host name. From there, other methods will return the host name and host address. A sample program is GetInetInfo.java:

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

public class GetInetInfo{
    public static void main(String[] args){
        if (args.length != 1) {
            System.err.println("Usage: GetInetInfo address");
            // System.exit(1);
            return;
        }

        InetAddress address = null;
        try {
            address = InetAddress.getByName(args[0]);
        } catch(UnknownHostException e) {
            e.printStackTrace();
            // System.exit(2);
            return;
        }
        System.out.println("Host name: " + address.getHostName());
        System.out.println("Host address: " + address.getHostAddress());
        // System.exit(0);
        return;
    }
} // GetInetInfo

This can be run by e.g. java jan.newmarch.name to give

	  Host name: jan.newmarch.name
	  Host address: 103.79.105.27
      

Get all DNS addresses

Note that it is only giving one address, with my setup (and probably yours) an IPv4 address. if you want to get all addresses registered with DNS, you need to use the method getAllByName() as in GetAllInetInfo.java:

import java.net.InetAddress;
import java.net.Inet4Address;
import java.net.UnknownHostException;

public class GetAllInetInfo{
    public static void main(String[] args){
        if (args.length != 1) {
            System.err.println("Usage: GetInetInfo address");
            // System.exit(1);
            return;
        }

        InetAddress[] addresses = null;
        try {
            addresses = InetAddress.getAllByName(args[0]);
        } catch(UnknownHostException e) {
            e.printStackTrace();
            // System.exit(2);
            return;
        }
        if (addresses.length > 0)
	    System.out.println("Host name: " + addresses[0].getHostName());
	for (int n = 0; n < addresses.length; n++) {
	    InetAddress address = addresses[n];
	    if (address instanceof Inet4Address)
		System.out.print("IPv4 address is ");
	    else
		System.out.print("Ipv6 address is ");
	    System.out.println(address.getHostAddress());
	}
        // System.exit(0);
        return;
    }
} // GetAllInetInfo

Then a typical run gives

	  $java GetAllInetInfo jan.newmarch.name
	  Host name: jan.newmarch.name
	  IPv4 address is 103.79.105.27
	  Ipv6 address is 2400:3740:200:d900:0:0:0:260
      

Get all addresses for an interface

An interface (such as eth0) may have multiple IPv6 addresses. There will usually be a link local address, probably a site local address and there could be many global addresses. A program to list them all is ListNets.java:

// From https://docs.oracle.com/javase/tutorial/networking/nifs/listing.html

import java.io.*;
import java.net.*;
import java.util.*;
import static java.lang.System.out;

public class ListNets {

    public static void main(String args[]) throws SocketException {
        Enumeration<NetworkInterface> nets = NetworkInterface.getNetworkInterfaces();
        for (NetworkInterface netint : Collections.list(nets))
            displayInterfaceInformation(netint);
    }

    static void displayInterfaceInformation(NetworkInterface netint) throws SocketException {
        out.printf("Display name: %s\n", netint.getDisplayName());
        out.printf("Name: %s\n", netint.getName());
        Enumeration<InetAddress> inetAddresses = netint.getInetAddresses();
        for (InetAddress inetAddress : Collections.list(inetAddresses)) {
            out.printf("InetAddress: %s\n", inetAddress);
        }
        out.printf("\n");
     }
}  

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Copyright © Jan Newmarch, jan@newmarch.name

" Network Programming using Java, Go, Python, Rust, JavaScript and Julia" by Jan Newmarch is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License .
Based on a work at https://jan.newmarch.name/NetworkProgramming/ .

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