ARPServer.java

import java.net.*;
import java.util.HashMap;

public class ARPServer {
    public static void main(String[] args) throws Exception {
        DatagramSocket ds = new DatagramSocket(9091);
        HashMap<String, String> arpTable = new HashMap<>();
        // Sample ARP Cache: IP -> MAC
        arpTable.put("192.168.1.1", "00:11:22:33:44:55");
        arpTable.put("192.168.1.2", "AA:BB:CC:DD:EE:FF");
        arpTable.put("192.168.1.3", "11:22:33:44:55:66");

        System.out.println("ARP Server running...");

        while (true) {
            byte[] buf = new byte[1024];
            DatagramPacket dp = new DatagramPacket(buf, buf.length);
            ds.receive(dp);

            String request = new String(dp.getData(), 0, dp.getLength());
            if (request.startsWith("REQUEST:")) {
                String ip = request.substring(8);
                String mac = arpTable.getOrDefault(ip, "Not Found");
                byte[] reply = ("REPLY: MAC for " + ip + " is " + mac).getBytes();
                ds.send(new DatagramPacket(reply, reply.length, dp.getAddress(), dp.getPort()));
                System.out.println("Sent reply for " + ip + ": " + mac);
            }
        }
    }
}