1. Internet and WWW Lecture 2

2. Network A network is a collection of computers connected together with special hardware (and software to manage the network). The purpose of a network is to share resources, which can be both physical (hardware) and digital (software and data). E.g. Atrium or this room-share printers and software

3. Client/Server model Client requests a service Server provides the service

4. LAN/MAN/WAN Local Area Network – communication in small area such as room or building Cables or wireless Metropolitan Area Network – links 2 or more LANs within a city Wide Area Network – communication over a larger distance Fiber optic cable Leased phone lines

5. Protocols Protocols are rules that must be followed so the computers on a network can communicate. If different networks use different technologies and protocols, they are incompatible. (Like 2 toddlers talking with neither listening.)

6. Networks Companies and organizations independently developed networks that were incompatible. They (especially the U.S. government) felt a need for internetworking – communication between networks. The Department of Defense funded research on how to achieve internetworking.

7. Robust internetwork The military had a particular perspective on internetworking: There was a fear of creating a network that would be dependent on a central "super computer" or central server. That was dangerous, because a central computer might be wiped out during a military attack.

8. Robust internetwork So the research was oriented towards designing a robust internetwork that would remain functional even if parts of the network would be - literally - destroyed in war. The Internet developed as a decentralized distributed network, with many servers.

9. ARPANet ARPANet was funded by the Dept of Defense for communications it needed to be resistant to attack or mechanical failure Eventually it became the Internet

10. The Internet The Internet is a vast, international network of networks the physical connections between computers vary, but the overall effect is that computers around the world can communicate and share resources

11. The Internet The Internet traces its roots back to 1969, when the U.S. government sponsored the first long-distance computer network Starting with only 4 computers, the network would eventually evolve into today's Internet

12. Robust Internet The success of this design was evident on 9/11. Phone service was wiped out in lower Manhattan, but Internet access was available in the unharmed buildings. Many people could not contact their family or friends by phone or cell phone, but were able to send email messages.

13. Communications protocol The major product of the research on internetworking was the development of a standard for a communications protocol to interconnect different networks.

14. Communications protocol A communications protocol is a standardized set of rules regulating communication between computers. (In foreign policy, protocol governs rules of interactions with foreign governments.)

15. TCP/IP The communications protocol used on the Internet is called TCP/IP. TCP/IP software enables a computer to communicate with other computers on the Internet. Any computer that is connected to the Internet must have TCP/IP software running on it.

16. Packet Switching In a packet-switching network, messages to be sent over the network are first broken into small pieces known as packets These packets are sent independently to their final destination TCP ( Transmission Control Protocol) breaks data into packets at the source and reassembles them at the destination

17. Advantages of Packets  Sending information in smaller units increases the efficient use of connections  Large messages can't monopolize the connection  Transmitting packets independently allows the network to react to failures or network congestion  routers (computers that direct the flow of messages) can recognize failures or congestion and reroute the packet around trouble areas

18. Advantages of Packets  Breaking the message into packets can improve reliability  Since the packets are transmitted independently, it is likely that at least part of the message will arrive (even if some failures occur within the network)  Software at the destination can recognize which packets are missing and request retransmission

19. IP addressing The cornerstone of TCP/IP is IP addressing. Every computer that is permanently connected to the Internet is given a unique number, called an IP address. (Similar idea to the way Social Security numbers are used to identify people.) Format of IP address - 146.245.1.4 - 4 groups of numbers, separated by periods. Each number 0- 255.

20. IP addressing The total number of IP addresses is 256 4. (0-255 each) Brooklyn College has IP addresses beginning with 146.245.... sci network is 146.245.1.x its network is 146.245.2.x There can be at most 256 computers on the sci network.

21. IP Internet Protocol (IP) relies on each computer having a unique identifier (known as an IP address) Each computer is assigned an IP address by its Internet Service Provider (ISP)

22. Datagram A datagram is a packet together with its routing information. Analogous to a letter inside its addressed envelepe.

23. TCP/IP Transmission Control Protocol (TCP) controls the method by which messages are broken down into packets and then reassembled when they reach their final destination Internet Protocol (IP) concerned with labeling the packets for delivery and controlling the packets’ paths from sender to recipient Routers are computers that receive packets, access the routing information, and pass the packets on toward their destination

24. DNS People don't like to have to remember numbers (e.g. soc sec #). We like to use names. The DNS - Domain Name System is used to give names to computers on the Internet. e.g. mail.sci.brooklyn.cuny.edu The parts are separated by periods. There is NO correspondence between these words and the numbers in the IP address. There may be more or less than 4 parts to a DNS name. IP addresses always have 4 parts.

25. DNS A DNS name can be "read" by going from the most specific to the more general, similar to the way a mailing address works. We might send mail to John Doe 123 Main Street Smalltown, ST

26. DNS The DNS name mail.sci.brooklyn.cuny.edu refers to a computer called "mail" which is part of the "sci" network, which is part of the Brooklyn College network, which is part of the CUNY system, which is an educational institution.

27. Top-level domains (TLD) The top-level domains are categories education (.edu) government (.gov) commercial (.com) organizations (.org) Military (.mil) Network support (.net)

28. Top-level domains There are country codes that are used as top-level domains, usually 2- characters.ca for Canada.jp for japan.ru for Russia.il for Israel.

29. DNS->IP Internet software automatically converts DNS names into IP addresses. When we specify a location, the software looks up the IP address and then initiates the connection. (Analogy: CEO asks a secretary to get someone on the phone. The secretary first looks up the phone number and then makes the phone connection.)

30. History of the Web The World Wide Web is a multimedia environment in which documents can be seamlessly linked over the Internet It was proposed by Tim Berners-Lee at the European Laboratory for Particle Physics (CERN) in 1989 It was designed to facilitate sharing information among researchers located all over Europe and using different types of computers and software

31. WWW Berners-Lee's design of the Web integrated two key ideas: Hypertext (documents with interlinked text and media--images and links to other pages the distributed nature of the Internet pages can be stored on machines all across the Internet, known as Web servers

32. World Wide Web (WWW) Like Internet communications, the Web relies on protocols to ensure that pages are accessible to any computer HyperText Transfer Protocol (HTTP) defines how messages are exchanged between browsers and servers (to be defined). The prefix http:// in a URL specifies that the HTTP protocol is to be used in communicating with the server.

33. Net and Web The World Wide Web is a collection of software that spans the Internet and enables the interlinking of documents and resources WWW uses the HTTP protocol. Many different services run “over” the Internet Services such as FTP, Telnet and email were established long before the WWW and can be used independently of the WWW

34. Internet ≠ World Wide Web The Internet could exist without the Web and did, in fact, for many years The Web couldn't exist without the Internet WWW runs over the internet. The Internet is the underlying communication network that executes the Web software

35. Web Page A Web page is a text document that contains additional formatting information in a language called HTML (HyperText Markup Language) which we will learn. A Web browser is a program that fetches an HTML file, interprets its content, and displays the page the way we like to see it. e.g. Internet Explorer

36. Web Server A Web server is an Internet-enabled computer that stores Web pages and executes software for providing access to the pages when you request a Web page, the browser sends a request over the Internet to the server the server locates the specified page and sends it back to your computer The browser interprets and displays the page

37. Web Addresses Web pages require uniform names to locate and identify them uniquely each page is assigned a Uniform Resource Locator (URL) URL's are commonly referred to as Web addresses the different parts of the Web address provide information for locating the page

38. URL - uniform resource locator http://www.sci.brooklyn.cuny.edu/~co gan First part - http:// - specifies the protocol to be used when retrieving this resource. Web browsers fill it in, because http is the default.

39. URL Second part - DNS name of server - www.sci.brooklyn.cuny.edu Third part - name or path to the file on the server. ~cogan

40. URL

41. Viewing Local Web Pages a Web browser can be used to view pages stored on the same computer can go through the File menu to select the local page, or can enter the File location in the address box (without the http prefix) this feature is handy when developing Web pages can create a Web page and view it in the browser before uploading to a server

42. Email address An email address is Account name @ DNS name