TCP/IP协议的四层模型，从底层到高层分别是:网络接口层、网络层、传输层、应用层。IP属于网络层，TCP/UDP属于传输层。IP定位对方的主机名，端口号对应接受数据的应用程序。
本地回环地址：127.0.0.1 主机名：localhost 有效端口：0~65535， 其中0~1024为系统保留端口。
IP地址由InetAddress类表示。没有构造函数，通过静态方法得到实例。
UDP：将数据及源和目的封装成数据包，不需要建立连接；每个数据包的大小限制在64k内；因无连接，是不可靠协议；速度快。
TCP：建立连接，形成传输数据的通道；在连接中进行大量数据传输；通过三次握手完成连接，是可靠协议；必须建立连接，效率稍低。
Socket：网络之间的通信其实就是Socket之间的通信，Socket之间通过IO进行传输。
DatagramSocket：UDP使用的Socket接口，是本地的接口；发送、接受的单位为DatagramPacket。注意：这里DatagramPacket里面有get/setPort的操作，其中set为设置此包的目的端口号，而get有两种情况，一种是未发送的时候得到刚才set的端口号，一种是在接收端得到发送端的Socket的端口号（发送端DatagramePacket的设置的端口号）。所以getPort是Packet的得到Port的方法，而getLocalPort是Socket得到Port的方法。
测试UDP传输简单的数据：
1 package net; 2 import java.net.DatagramPacket; 3 import java.net.DatagramSocket; 4 import java.net.InetAddress; 5 //主线程为server端接受，子线程为client发送 6 public class TestUDP { 7 public static void main(String[] args) { 8 // TODO Auto-generated method stub 9 UDPClient udpClient = new UDPClient();10 Thread t1 = new Thread(udpClient);11 t1.start();12 UDPServerFunc();13 }14 static void UDPServerFunc(){15 try {16 DatagramSocket dsServer = new DatagramSocket(7777);17 System.out.println("Server DatagramSocket Port"+dsServer.getLocalPort());18 byte[] buf = new byte[1024];19 DatagramPacket dpServer = new DatagramPacket(buf, 0, buf.length);20 dsServer.receive(dpServer);21 System.out.println(dpServer.getAddress().getHostAddress() + " :: "22 + new String(dpServer.getData(),0,dpServer.getLength()));23 System.out.println("Server DatagramPacket Port"+dpServer.getPort());24 } catch (Exception e) {25 e.printStackTrace();26 }27 }28 }29 class UDPClient implements Runnable{30 @Override31 public void run() {32 try {33 DatagramSocket dsClient = new DatagramSocket(6666);34 System.out.println("Client DatagramSocket Port"+dsClient.getLocalPort());35 byte[] buf = "hello this is UDPTest".getBytes();36 DatagramPacket dpClient =37 new DatagramPacket(buf, buf.length,InetAddress.getByName("localhost"), 7777);38 Thread.sleep(1000);39 System.out.println("Client : "+"wakeup");40 System.out.println("Client DatagramPacket Port"+dpClient.getPort());41 dsClient.send(dpClient);42 } catch (Exception e) {43 e.printStackTrace();44 }45 }46 }
UDP是没有“方向”的，所以用DatagramePacket既发送数据，又接收数据。但是TCP是有“方向”的连接，因此用两个类Socket 、ServerSocket分别表示客户端和服务器端， TCP先形成Socket的“通路”，然后再通过IO流在这条通路上进行数据的传输。即ServerSocket先接收到客户端的Socket，然后调用此Socket中的IO流实现数据的流传输。
1 package net; 2 //主进程中为server 子进程中为client 3 import java.io.BufferedReader; 4 import java.io.BufferedWriter; 5 import java.io.IOException; 6 import java.io.InputStream; 7 import java.io.InputStreamReader; 8 import java.io.OutputStream; 9 import java.io.OutputStreamWriter;10 import java.net.BindException;11 import java.net.ServerSocket;12 import java.net.Socket;13 import java.net.UnknownHostException;14 15 public class TestTCP {16 public static void main(String[] args){17 try {18 ServerSocket ssever = new ServerSocket(7777);19 System.out.println("ServerSocket Port:" + ssever.getLocalPort());20 Thread tclient = new Thread(new TCPClient());21 tclient.start();22 23 Socket s1 = ssever.accept();24 InputStream serverIn = s1.getInputStream();25 OutputStream serverOut = s1.getOutputStream();26 System.out.println("Server : "+"connected host : "+"addr --"+s1.getInetAddress().getHostAddress()27 +";port --"+s1.getPort());28 BufferedReader serverBR = new BufferedReader(new InputStreamReader(serverIn));29 BufferedWriter serverBW = new BufferedWriter(new OutputStreamWriter(serverOut));30 System.out.println("ready");31 // String serverStr ="aaaaaaaa";//测试数据用32 String serverStr = serverBR.readLine();33 while (true) {34 if (serverStr.equalsIgnoreCase("over")) {35 s1.close();36 break;37 }38 System.out.println("Server receive string : " +serverStr);39 Thread.sleep(500);40 serverBW.write(serverStr.toUpperCase());41 serverBW.newLine();42 serverBW.flush();43 serverStr = serverBR.readLine();44 }45 46 } catch (Exception e) {47 System.out.println("main problem");48 }49 }50 }51 class TCPClient implements Runnable {52 @Override53 public void run() {54 try {55 Thread.sleep(1000);56 Socket sclient = new Socket("localhost", 7777);57 System.out.println("Client Socket Port"+sclient.getLocalPort());58 BufferedReader consleBR = new BufferedReader(new InputStreamReader(System.in));59 BufferedWriter clientBW = new BufferedWriter(new OutputStreamWriter(sclient.getOutputStream()));60 BufferedReader clientBR = new BufferedReader(new InputStreamReader(sclient.getInputStream()));61 62 String consoleStr = consleBR.readLine();63 String receiveStr = null;64 while(true){65 if (consoleStr.equalsIgnoreCase("Over")) {66 clientBW.write("over");67 clientBW.newLine();68 clientBW.flush();69 break;70 }71 clientBW.write(consoleStr);72 clientBW.newLine();73 clientBW.flush();74 // SOP.sop("console : "+consoleStr);75 receiveStr = clientBR.readLine();76 System.out.println("Client receive string : "+receiveStr);77 consoleStr = consleBR.readLine();78 }79 clientBR.close();80 clientBW.close();81 consleBR.close();82 83 } catch (UnknownHostException e) {84 System.out.println("UnknownHostException ...");85 } catch (InterruptedException e1) {86 System.out.println("InterruptedException ...");87 }catch (BindException e2) {88 System.out.println("BindException ...");89 }catch (IOException e) {90 System.out.println("IOEcxeption ... ");91 }92 }93 94 }
来源：https://www.cnblogs.com/zr-714/archive/2012/03/31/2426789.html