THE INTERNET
   
1．What is the Internet？
   
　　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 Internet evolved in part from ARPAnet．
    ARPAnet is abbreviated from“Advanced Research Projects Agency Network”．Landmark packet-switching network established in 1969．ARPAnet was developed in the 1970s by BBN and funded by ARPA[1]．It eventually evolved into the Internet．The term ARPAnet was officially retired in 1990．
   
2．Internet services
   
　　From the user’s point of view，a TCP/IP internet[2] appears to be a set of application programs that use the network to carry out useful communication tasks．We use the term interoperability to refer to the ability of diverse computing systems to cooperate in solving computational problems．Internet application programs exhibit a high degree of interoperability．Most users that access the Internet do so merely by running application programs without understanding the TCP/IP technology，the structure of the underlying internet，or even the path the data travels to its destination；they rely on the application programs and the underlying network software to handle such details．Only programmers who write network application programs need to view the internet as a network and need to understand some of the technology．
    The most popular and widespread Internet application services include：

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．The server and client communicate through a transfer protocol，usually the HyperText Transfer Protocol（HTTP）[4]．The server then accesses a Web page using a Uniform Resource Locator（URL）．Search engines are available to simplify access by enabling users to enter search criteria on a topic and have several URLs returned for Web pages that pertain to the desired information[5]．
　　Browser is a GUI-based hypertext client application，such as Internet explorer（IE），Mosaic，and Netscape Navigator，used to access bypertext documents and other services located on innumerable remote servers throughout the WWW and Internet．

2）Electronic mail

    Electronic mail allows a user to compose memos and send them to individuals or groups．Another part of the mail application allows users to read memos that they have received．Electronic mail has been so successful that many Internet users depend on it for normal business correspondence[6]．Although many electronic mail systems exist，using TCP/IP makes mail delivery more reliable because it does not rely on intermediate computers to relay mail messages[7]．A TCP/IP mail delivery system operates by having the sender’s machine contact the receiver’s machine directly．Thus，the sender knows that once the message leaves the local machine，it has been successful received at the destination site．
　　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．Although users sometimes transfer files using electronic mail，mail is designed primarily for short text messages．The TCP/IP protocols include a file transfer application program that allows users to send or receive arbitrarily large files of programs or data．For example，using the file transfer program，one can copy from one machine to another a large database containing satellite images，a program written in Pascal or C＋＋，or even an English dictionary．The system provides a way to check for authorized users，or even to prevent all access．Like mail，file transfer across a TCP/1P internet is reliable because the two machines involved communicate directly，without relying on intermediate machines to make copies of the
file along the way．

4）Telecommunication network（Telnet）

　　Telnet is a standard terminal emulation protocol in the TCP/IP protocol stack．Telnet is used for remote terminal connection，enabling users to log in to remote systems and use resources as if they were connected to a local system．
　　Remote login allows a user sitting at one computer to connect to a remote machine and establish an interactive login session．The remote login makes it appear that a window on the user’s screen connects directly to the remote machine by sending each keystroke from the user’s keyboard to the remote machine and displaying each character the remote computer prints in the user’s window[10]．When the remote login session terminates，the application returns the user to the local system．

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