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 @Override
31 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 @Override 53 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 }