Trail: Custom Networking
Lesson: Programmatic Access to Network Parameters

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Retrieving Network Interfaces

The NetworkInterface class has no public constructor. Therefore, you cannot just create a new instance of this class with the new operator. Instead, the following static methods are available so that you can retrieve the interface details from the system: getByInetAddress(), getByName(), and getNetworkInterfaces(). The first two methods are used when you already know the IP address or the name of the particular interface. The third method, getNetworkInterfaces() returns the complete list of interfaces on the machine.

Network interfaces can be hierarchically organized. The NetworkInterface class includes two methods, getParent() and getSubInterfaces(), that are pertinent to a network interface hierarchy. The getParent() method returns the parent NetworkInterface of an interface. If a network interface is a subinterface, getParent() returns a non-null value. The getSubInterfaces() method returns all the subinterfaces of a network interface.

The following example program lists the name of all the network interfaces and subinterfaces (if any exist) on a machine.

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

public class ListNIFs 
{
    public static void main(String args[]) throws SocketException {
        Enumeration<NetworkInterface> nets = NetworkInterface.getNetworkInterfaces();
        
        for (NetworkInterface netIf : Collections.list(nets)) {
            out.printf("Display name: %s\n", netIf.getDisplayName());
            out.printf("Name: %s\n", netIf.getName());
            displaySubInterfaces(netIf);
            out.printf("\n");
        }
    }

    static void displaySubInterfaces(NetworkInterface netIf) throws SocketException {
        Enumeration<NetworkInterface> subIfs = netIf.getSubInterfaces();
        
        for (NetworkInterface subIf : Collections.list(subIfs)) {
            out.printf("\tSub Interface Display name: %s\n", subIf.getDisplayName());
            out.printf("\tSub Interface Name: %s\n", subIf.getName());
        }
     }
}

The following is sample output from the example program:

Display name: bge0
Name: bge0
    Sub Interface Display name: bge0:3
    Sub Interface Name: bge0:3
    Sub Interface Display name: bge0:2
    Sub Interface Name: bge0:2
    Sub Interface Display name: bge0:1
    Sub Interface Name: bge0:1

Display name: lo0
Name: lo0

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