Internet is the largest global internetwork，connecting tens of thousand of networks  worldwide and having a“culture”that focuses on research and standardization based on real-life use．Many leading-edge network technologies come from the Internet community．The most popular and widespread Internet application services include：

    1.WWW（word wide web）　2.Electronic mail　3.File Transfer Protocol（FTP）

1）WWW（word wide web）

　　WWW is a large network of Internet servers providing hypertext and other services to terminals[3] running client applications such as a browser．

    WWW enables users to search，access，and download information from a worldwide series of networked servers where information is dynamically interlinked．A Web client passes a user’s request for information to a server，usually by way of a Web browser．

　　Browser is a GUI-based hypertext client application，used to access bypertext documents and other services located on innumerable remote servers throughout the WWW and Internet．

2）Electronic mail

　　There are two kinds of E-mail protocol used in the Internet．One is Simple Mail Transfer Protocol（SMTP）which accepts incoming connections and copies messages from them into the appropriate mailboxes[8]．Another is Post Office Protocol-3（POP3）which fetches E-mail from the remote mailbox and stores it on the user’s local machine to be read later．

3）File Transfer Protocol（FTP）

　　FTP is an application protocol，part of the TCP/IP protocol stack，used for transferring files between network nodes．The TCP/IP protocols include a file transfer application program that allows users to send or receive arbitrarily large files of programs or data．

4）Telecommunication network（Telnet）

　　Telnet is a standard terminal emulation protocol in the TCP/IP protocol stack．