Network programming enables communication between software applications across different devices over a network. Whether you're developing a chat application, implementing web services, or building client-server systems, understanding network programming is essential in today's connected world. This chapter explores the fundamental concepts, protocols, models, and programming techniques used to build networked applications, with practical implementation using Java (and references to Python/C for parallel understanding). It also covers low-level socket programming as well as higher-level abstractions, providing a comprehensive foundation for real-world application development.

Before diving into code, it's crucial to understand how data flows between computers.

What is a Network?

A network is a collection of interconnected devices (computers, servers, routers) that communicate to share data and resources.

Types of Networks:

• LAN (Local Area Network)
• WAN (Wide Area Network)
• MAN (Metropolitan Area Network)
• PAN (Personal Area Network)

Network Models

• OSI Model (7 layers)
• Physical, Data Link, Network, Transport, Session, Presentation, Application
• TCP/IP Model (4 layers)
• Network Access, Internet, Transport, Application

Protocols

• TCP (Transmission Control Protocol) – reliable, connection-oriented
• UDP (User Datagram Protocol) – faster, connectionless
• HTTP/HTTPS, FTP, SMTP, DNS, etc.

Socket programming provides an API for communication between machines.

What is a Socket?

A socket is one endpoint of a two-way communication link between two programs running on the network.
- Socket = IP address + Port number
- Types: Stream (TCP), Datagram (UDP)

Java Socket Classes

• java.net.Socket – client side (TCP)
• java.net.ServerSocket – server side (TCP)
• java.net.DatagramSocket, java.net.DatagramPacket – for UDP

TCP Server Example

import java.net.*;
import java.io.*;
public class TCPServer {
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = new ServerSocket(5000);
System.out.println("Server started...");
Socket clientSocket = serverSocket.accept();
System.out.println("Client connected.");
BufferedReader in = new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
String message = in.readLine();
System.out.println("Received: " + message);
out.println("Echo: " + message);
in.close();
out.close();
clientSocket.close();
serverSocket.close();
}
}

TCP Client Example

import java.net.*;
import java.io.*;
public class TCPClient {
public static void main(String[] args) throws IOException {
Socket socket = new Socket("localhost", 5000);
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new
InputStreamReader(socket.getInputStream()));
out.println("Hello Server!");
System.out.println("Server response: " + in.readLine());
in.close();
out.close();
socket.close();
}
}

UDP Server Example

import java.net.*;
public class UDPServer {
public static void main(String[] args) throws Exception {
DatagramSocket socket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
DatagramPacket receivePacket = new DatagramPacket(receiveData,
receiveData.length);
socket.receive(receivePacket);
String message = new String(receivePacket.getData()).trim();
System.out.println("Received: " + message);
socket.close();
}
}

UDP Client Example

import java.net.*;
public class UDPClient {
public static void main(String[] args) throws Exception {
DatagramSocket socket = new DatagramSocket();
byte[] sendData = "Hello UDP Server".getBytes();
InetAddress IPAddress = InetAddress.getByName("localhost");
DatagramPacket sendPacket = new DatagramPacket(sendData,
sendData.length, IPAddress, 9876);
socket.send(sendPacket);
socket.close();
}
}

Ports

• Range: 0–65535
• Well-known ports: 0–1023 (e.g., HTTP – 80, FTP – 21)

IP Addressing

• IPv4: e.g., 192.168.1.1
• IPv6: longer format to support more devices

DNS and Domain Names

Domain Name System maps domain names to IP addresses.

To handle multiple clients simultaneously:

class ClientHandler extends Thread {
private Socket clientSocket;
public ClientHandler(Socket socket) {
this.clientSocket = socket;
}
public void run() {
try {
BufferedReader in = new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(),
true);
String inputLine;
while ((inputLine = in.readLine()) != null) {
out.println("Echo from server: " + inputLine);
}
in.close();
out.close();
clientSocket.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}

And in the main server:

ServerSocket serverSocket = new ServerSocket(5000);
while (true) {
Socket socket = serverSocket.accept();
new ClientHandler(socket).start();
}