The Internet Advanced Research Projects Agency Network (ARPANET) How the Internet Works Transport Control Protocol (TCP)

Transcription

1 The Internet, Intranets, and Extranets 1

2 The Internet The Internet is a collection of interconnected network of computers, all freely exchanging information. These computers use specialized software to communicate between themselves. Research firms, colleges, universities, schools, businesses use one or more host computers to make some of the documents and software available to the public. Advanced Research Projects Agency Network (ARPANET) ARPANET is the ancestor of Internet. The network was initiated by the Department of Defense (DOD) to facilitate communication between various military research organizations, including DODfunded research universities. How the Internet Works The Internet works in the client-server methodology. When a client computer requests data from the Internet, the data packet travels from one computer to another in the network, until it reaches the server computer where the data is stored. The response follows the same computer-to-computer path back to the person who made the request. Data is transmitted between computers in chunks called packets, each of which carries addresses of its sender and its receiver. The set of conventions used to pass packets from one host to another is known as the Internet Protocol (IP). Transport Control Protocol (TCP) A protocol that operates at the transport layer of the OSI model and is used in combination with Internet protocol by most Internet applications. Thus it is commonly known as TCP/IP. 2

3 Backbone The high-speed, long distance communications link that connects the internal network to the Internet. Backbone Accessing the Internet There are three ways one can connect to the Internet: LAN Server SLIP/PPP ISP 3

4 Connection via LAN Servers In a business with a local area network (LAN), a dedicated server computer with a special software, is usually used to access the Internet. In a LAN, each computer is already installed with a network card. The software installed in a client computer allows connection to the Internet through the Internet server. The server provides many other services such as Web hosting, FTP, Telnet, and etc. Microsoft Internet Information Server (IIS) is one of this software. Connection via SLIP/PPP This approach requires a modem and the TCP/IP protocol software plus Serial Line Internet Protocol (SLIP) or Point to Point Protocol (PPP). SLIP and PPP are two communication protocols that transmit data packets over telephone lines, allowing dial-up access to the Internet. Users must also have an Internet service provider that lets them dial into a SLIP/PPP server through the modem. Connection via an On-Line Service This approach requires a modem and software to dial to a highperformance internet server computer installed by a provider such as America Online, Prodigy, and Microsoft Network. These services usually require sign-up procedures. Internet Service Provider (ISP) Any company that provides individuals or companies with access to the Internet. ISPs do not offer the extended informational services provided by the on-line services. There are thousands of ISPs, but the large ones are AT&T, MCI, GTE, Sprint, and SW Bell. Need an account with the ISP and software that links via TCP/IP. Some ISPs also offer Internet connection via satellite. These services offer high-speed connection, which is about Kbps as compared to Kbps for telephone lines. 4

5 The World Wide Web In 1989, Tim Berners-Lee, a computer scientist working at CERN, the European Particle Physics Laboratory, came up with a better way for physicists to communicate. The idea was to use hypertext documents in an electronic form that are linked together in some fashion, and each retrievable by clicking on a hyperlink. The idea became reality in 1992 in the term World-Wide Web evolved. The World-Wide Web, also called Web, now handles hypermedia files such as text, graphics, audio, and video files. The Internet provides the network and the Web provides the method of storing and retrieving its documents. The Web is a menu-based system that uses the client/server model. Web Site (Web Server) This refers to the server computer where hypermedia files are stored, which are accessed by a client computer in the Internet, using a browser. Uniform Resource Locator (URL) Each Web server on the Internet is assigned a unique address called a URL, to identify it from other hosts. Example: Here http specifies the access method and tells the browser software to access a file using the HyperText Transport Protocol. This is the primary method for interacting with the Internet. Other access methods include ftp (File transfer Protocol) for transferring files between computers, telnet for logging onto a remote computer, news for bulletin boards or newsgroups, and gopher for accessing via Gopher menu. The www part of the address signifies that the address is associated with the World Wide Web service. 5

6 The thomson.com part of the address is the domain name that identifies the Internet host or server site. It always has at least two parts separated by dots. All countries except the USA, the rightmost part is the country code. Within USA, the rightmost part identifies the host affiliation such as educational (edu), governmental (gov), business (com), and so on. 6

7 Web Browser Software that creates a unique hypermedia-based menu on a client computer screen and provides a graphical interface to the Web. It is used to access information from a Web server. Netscape Navigator and Microsoft Internet Explorer are two widely used browsers. Web Page It refers to the hypermedia files stored in a Web server. A Web page is identified by a unique address through the unique Web site address. Home Page It refers to the first page of a Web site. Other pages at the site can be reached from the home page through hyperlinks. Example: Hypertext Markup Language (HTML) HTML consists of a standard set of codes that are used develop a Web page. It contains a set of tags that let the browsers know how to format the when displayed. It contains tags for header, body, list, table, row, image, and so on. 7

8 Applet A small program, typically written in Java language, embedded in Web pages that can show multiple images in the same content or play music when clicked. Applets give dynamic nature of the Web pages. Java Java is an object-oriented programming language developed by Sun Microsystems. It is like C++ program, but it runs on any computer hardware and software platform (Windows, Unix, MacOS, OS/2). Commonly Java is used to embed a dynamic picture in a Web page. In this case, Java applets are embedded in a Web page using a <APP> tag. Java can also used to write a program for an information system. As it is independent of the platform, it is portable. Search Engine A search engine catalogs Web pages by keywords. It is like cataloging books in a library by subject, title, or author. A search engine also catalogs Web pages by subjects. A search engine is a program that reads each page in the World Wide Web and looks for certain key words. When the word is found, it catalogues the word, number of hits of the word in the page, and the address of the Web page, in a file or database. Placing indexes to the common words of human interest makes a subject directory. As there are thousands and thousands of Web servers in the World Wide Web, each time it takes several months for a program to crawl through the Web. As new servers are added in the Web, the cataloging is a constantly evolving process. Thus, when searched by one or more keywords, the program immediately shows the links to the most probable pages. AltaVista, Yahoo!, Excite, Infoseek, Google, and Lycos are common search engines. 8

9 Web Server Software As it was mentioned above, a server computer is used to host or provide services to a client computer in the Internet. The components of an Internet server software are shown below with brief description of each. 9

10 Internet Services The Internet, alongwith the World-Wide Web provides many services to a computer user. The most commonly used services of the Internet are: 10

11 Voice-Over-Internet Phone (VOIP) Users in a computer network can route phone and fax transmission over the network alongwith data transmission. Telephones are connected to a PBX, which in turn is connected to a VOIP gateway and then a router. This arrangement is installed at both ends of the communication link. The VOIP gateway converts voice into data packets and the router shunts them onto the network. When the packet reaches the destination gateway, the message is depacketized, converted back into voice and sent out via local telephone lines. fax transmissions over the same network they use for da 11

12 Intranets and Extranets Intranet An internal corporate network built using the Internet and World Wide Web technology. It allows employees of an organization to gain access to corporate information just like they access the Web. Corporations block public access to the internal Web pages using a device called firewall. Employees can venture out to the Internet, but unauthorized users can not come in. Large companies are using intranet to communicate with their employees to reduce paper document transmittal and storage. Extranet An extranet is a network that links some of the intranet resources of a company with other organizations and business partners. Extranet enable customers, suppliers, contractors, consultants, and others to access selected intranet Web sites and other company databases. Firewall A method of preventing unauthorized access between a company s computers and the Internet. A device is used that sits between the internal network and the outside Internet and looks at the header of a data packet, and makes a decision to keep data inside the network or forward outside. 12

13 Virtual Private Network A virtual private network (VPN) is a secure connection between two points across the Internet. VPNs transfer information by encapsulating (coded for security) IP packets and sending the packets over the Internet, a practice called tunneling. VPN is useful in communicating files between two branches of a business. 13

14 Internet Issues Management Issues Although Internet is a huge, global network, it is managed at the local network level of a business. No centralized governing body controls the Internet. Thus varieties of information (good, bad, useful, ugly) are published in the Web. This might change in the future through regulations. Privacy and Security Internet is a network of free-flowing information. When secured information needs to be sent over the Internet, it needs to be converted to a code such that no one can understand the content of the original information. Cryptography is the process of converting a message into a secret code and changing the encoded message back to regular text. The conversion of original message to a secret code is called encryption. The conversion of encoded message back to original text is called Decryption. A cryptosystem is a software package that uses an algorithm, or mathematical formula, plus a key to encrypt and decrypt messages. 14