CS 170 Lecture 3: Computer Networks & Information on the Internet Kristian Stout firstname.lastname@example.org Office: IN: Livingston Student Center, A lounge near ?the rock? and the commuter lounge. Office Hours: TTH 5:30 ? 6:30 Computer Networks A computer network is an interconnection of a group of computers and related equipment. Need and Origin Arpanet was the first network as we typically think of them. Types Area covered ( WAN, LAN, VPN, etc ) Communication media ( Fiber, Ethernet, Wireless, etc ) Administration (Intranet, Extranet, Internet) Protocols and Hardware TCP/IP, Ethernet etc Switches, Routers, NIC Cards etc Communication Types Based on when sender and receivers are active Synchronous Two active parties Asynchronous Two parties at different tiems Based on number of receivers Broadcast communication Multicast communication Point-to-point communication The Internet ?totality of all the wires, fibres, switches, routers, satellite links and other hardware for transporting information between addressed computers.? The Client/Server Structure Server is the computer that on request provides some services Client is the computer that requests for some services from the server A client is not ALWAYS a client; sometimes it can be a server. Computers continually shift roles in this relationship. IP Addresses and Domain Names IP addresses: Each computer connected to the Internet is given a unique numerical address Series of four numbers separated by periods eg 188.8.131.52 Not in ipv6, though Domain Names: Human-readable symbolic names, based on domain hierarchy Human readable aliases of IP Addresses Top-level Domains Domain is a related group of networked computers Top-level domains appear in the last part of domain name: .edu educational institutions .org organizations .net networks .mil military .gov government agencies Mnemonic two-letter country designators such as .ca (Canada) ftp://tracer.cs.washington.edu DNS Servers The Domain Name System translates the human-readable names into IP addresses Sort of a ?map? between IP addresses and Domain Names Internet host knows the IP address of its nearest DNS server, a computer that keeps a list of domain names and corresponding IP addresses When you use a domain name to send information, your computer asks the DNS server to look up the IP address If the DNS server doesn't know the IP address, it asks a Root name server, which keeps the master list of name-to-address relationship DNS Servers Note: Client-Server roles flip back and forth Internet Protocol and Packet Switching TCP/IP (Transmission Control Protocol/ Internet Protocol) TCP/IP is the most common method of internet communication IP packets Data, destination and source IP address, sequence number Independence from media used for transmission IP Packets are independent from each other; each packet takes any available route to it?s destination The World Wide Web The World Wide Web is a subset of the Internet Typical Roles Web server: Computers programmed to send files to browsers running on other computers connected to the Internet Web client: Computers that consume information provided by servers. Sometimes, the browser software is called the client. Web page: A document containing text and graphics (created in HTML) that can be accessed through a web browser on the internet. Web Site: A collection of web pages and directories that form a logical whole. (there are other parts of a web site, but this definition is a good start) Requesting a Web Page Universal Resource Locator (URL) has three main parts Protocol: http:// Server computer's name: Server's IP address given by the domain hierarchy Page's pathname: Tells the server which file (page) is requested and where to find it. Domain name and IP conversion Pathname resolution http://localhost/galleries/gal100/pioneer.html Note: - Spaces in file names matter - Capitalization (case) matters And now ? Information Information, a definition Information is the result of processing, manipulating and organizing data in a way that adds to the knowledge of the person receiving it Some Examples Telephone Directory Sociological Statistics Paintings Finding information requires a repository of data and indexes enabling fast retrieval Organizing Information Need for Structure Vastness of Data Ease of Use Hierarchy Structure A Hierarchy is a system of ranking and organizing. functions by exclusion Classification Multilevel Disjoint or Overlapping The biological system of classification is disjoint MSN.com is overlapping Hierarchies - The top of a hierarchy is the most general level, eg categories - sub-categories are attached to the category that provide greater elaboration for the general - eventually the hierarchy ends at either a single member, or a small, easily ?thinkable? collection of members. Search Engines Search Engine: Collection of computer programs that help us find information on the web. Two Basic Parts Crawler Query Processor Crawlers When a crawler visits a website: 1. First identifies all the links to other Web pages on that page 2. Checks its records to see if it has visited those pages recently 3. If not, adds them to list of pages to be crawled 4. Records in an index the keywords used on a page (appear in the titles, the body, or in anchor text) - The ?invisible web? Query Processor Gets keywords from user and looks them up in its index Stems the query Returns pages ranked by relevance Readings for Next Lecture Chapter 4 HTML (pp 90 ? 99 )
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