Fluency with Information Technology 7th Edition Chapter 3 The Basics of Networking

Learning Objectives 1.1 Correctly identify synchronous vs. asynchronous, broadcast vs. point-to-point communication technologies in the client/server structure of the Internet 1.2 Explain the roles of Internet addresses, domain names, and DNS servers in networking 1.3 Distinguish between TCP/IP and Ethernet protocols 1.4 Describe how computers are interconnected by ISPs, WANs, and LANs 1.5 Distinguish between the Internet and the World Wide Web 1.6 Explain file structure, and describe how to navigate a file hierarchy

1.1 Communication Types and the Structure of the Internet To understand the Internet we need to cover some basic communication vocabulary: Synchronous communication Asynchronous communication Broadcast Multicast Point-to-point

Definitions: Communication Activity Synchronous communication Both the sender and the receiver are active at the same time (think of talking on a telephone) Asynchronous communication The sending and receiving occur at different times (think of email and answering machines)

Definitions: Communication Receivers Another property of communication concerns the number of receivers: Broadcast communication: single sender and many receivers (radio and TV) Multicast: has many receivers, but usually a specific group (specialized topics) Point-to-point communication: one specific sender and one specific receiver (telephone call)

The Internet: Communication Properties The Internet supports point-to-point asynchronous communication The Internet is fast enough to mimic synchronous communication (like using a phone) Multicasting is also possible, allowing groups to communicate in chat rooms You can post video that can be accessed by anyone, as a form of broadcasting (compares with radio or television)

The Internet: A Communication Medium The Internet provides a general communication “fabric” linking all computers connected to it Computers and the network become a single, universal communication medium The Internet also becomes more effective with each additional computer added If x computers are already attached to the Internet, adding one more results in x potential new connections

Figure 3.1 Internet Schematic Diagram A schematic diagram of the Internet.

The Internet: Clients and Servers Most interactions over the Internet use the client/server interaction protocol: When you click a link, your computer gets the page for you, beginning the client/server interaction: Your computer is the client computer and the computer with the web page is the web server - the client gets services from the server When the page is returned, the operation is completed and the client/server relationship ends

Figure 3.2 Basic Client/Server Interaction The basic client/server interaction, as illustrated by the browser (client) requesting web pages provided by the web server

Client/Server Structure Fundamental to Internet interactions Only a single service request and response are involved The client/server relationship only lasts from the moment the request is sent to the moment the service is received (and can then be reinitiated) This approach means that the server can handle many clients at a time For example, between two consecutive client requests from your browser (getting a page and asking for another) that server could have serviced hundreds of other clients The server is busy only for as long as it takes to perform your request

Figure 3.3 Client/Sever Relationships Client/server relationships as they might develop over time.

Making the Client/Server Moment Last: Appearing to Stay Connected Even though users interact with a website through multiple brief exchanges, many sites give the appearance that these are part of a longer-lived (persistent) connection Example: when using a bank website, your login interaction must be associated with your transactions Example: when buying on-line, your purchases must be related to your shopping cart

Appearing to Stay Connected: Cookies and URL Parameters How do websites make the session seem persistent? Cookies: The server stores a small file on the client, which is returned with each request Contains enough information to associate the interactions URL parameters: Information is added to the URL Example: The URL created by a search: https://www.bing.com/search?q=the+internet&for m=EDGHPC&qs=PF&cvid=3d2711f651a04350938c 85fe7d28016c&cc=US&setlang=en-US

1.2 Networking: IP addresses, domain names, and DNS servers Each computer connected to the Internet is given a unique address called its IP address An IPv4 address is a series of four numbers (one byte each) separated by dots The range of each of these numbers (0–255) allows for billions of IP addresses IPv6 will allow for 128 bit addresses, much more than IPv4’s 32

Beyond IP Addresses: Domain Names A domain is related group of networked computers The Internet uses human-readable symbolic names for computers that are based on a hierarchy of domains Example: spiff.cs.washington.edu The name of the computer is spiff Which is part of the Computer Science and Engineering Department domain (cs) Which is part of the University of Washington domain (washington) Which is part of the educational domain (edu)

The Domain Hierarchy The spiff.cs.washington.edu example shows a hierarchy of domains, where each is a member of the next larger domain .edu is a peer of other top-level domains such as .com These domain names are symbolic and meaningful, making them easier to read than numbers (and easier to remember)

Top-Level Domains (TLDs) TLDs have been expanded from.com, .edu. .gov, and others to include biz, info, name, travel, and more The full list can be found at www.icann.org (ICANN is the Internet Corporation for Assigned Names and Numbers) There are also two-letter country designators: (ca (Canada), uk (United Kingdom), fr (France), de (Germany, as in Deutschland), etc.) These allow domain names to be grouped by their country of origin.

Figure 3.8 Two Views of the .edu Domain Hierarchy Two ways to think about the Internet domain hierarchy.

Domain Name System (DNS) Servers The Domain Name System (DNS) translates the hierarchical, human-readable names into IPv4 and IPv6 addresses Every Internet host knows the IP address of its nearest DNS name server Whenever the hierarchical symbolic name is used to send information to a destination, the client computer asks the DNS server to look up the corresponding IP address

DNS Lookup: Step-by-step Your computer asks the nearest DNS server to translate a name, like www.airandspace.si.edu That local server asks a root name server for the IP address of the edu authoritative name server (the administrator of each DNS server makes sure it has a list of root servers to get started) There are 13 root name servers Those 13 share the load Multiples prevent issues if one crashes Next it asks that edu server for the si authoritative name server Then it asks the si server for the airandspace server, which it asks for the IP address of www

1.3 TCP/IP and Ethernet Understanding the Transmission Control Protocol/Internet Protocol (TCP/IP) The Internet is like sending a novel to your publisher using postcards The novel is broken into small units that fit on a postcard The postcards are numbered to indicate where each belongs in the novel As each postcard is completed, it gets mailed

A TCP/IP Postcard Understanding the Transmission Control Protocol/Internet Protocol (TCP/IP) The Internet is like sending a novel to your publisher using postcards The novel is broken into small units that fit on a postcard The postcards are numbered to indicate where each belongs in the novel As each postcard is completed, it gets mailed

Postcards: Keep Those Messages Brief What a postcard to your publisher might look like.

TCP/IP: From Postcard to IP Packet Sooner or later, your publisher receives the postcards, but not necessarily in order The postcards don’t take the same route to get there, but once there, the cards can be put in order For TCP/IP, these postcards are IP packets They contain: one unit of information content, the destination IP, and their sequence number (which packet they are)

Transmitting Information: IP Packets Each message on the Internet is called an IP packet, sent to a particular IP address Each packet may take a different route to reach that address Because each independent packet can take a different route, congestion and service interruptions do not delay transmissions (packets can arrive out of order) A tool like traceroute can be used to find the route The TCP/IP protocol works under adverse conditions If traffic is heavy and the packet progress is slow, the protocol allows the packet to be thrown away If a packet is killed for whatever reason, the recipient will request a resend

WANs vs LANs: The Internet and Ethernet The Internet is a collection of wide area networks (WANs) that are not geographically close A collection of point-to-point channels IP packets must visit a sequence of computers before they reach their destinations

Local Area Networks (LANs) Connect computers which are geographically close Usually they can be linked by a single cable or pair of wires Ethernet is the main technology for local area networks Used for connecting all the computers in a lab or building

LANs and Ethernet Ethernet networks use wires, wire pairs, or optical fibers to connect, called the channel Engineers “tap” into the channel to connect a computer This allows it to send a signal or an electronic pulse or light flash onto the channel All computers, including the sender, can detect the signal

Ethernet: Not a Postcard, but a Party An Ethernet network looks a lot like a party, with a computer everywhere you’d fins a person: A group of computers is standing around at a party telling stories While one computer is telling a story, every computer is listening When the story is over, there may be a pause before the next computer speaks Then, another computer starts talking and the cycle starts again

Having a Good Ethernet Party, Wish You Were Here At an Ethernet party, all “people” are equal No computer has a more important status Every computer speaks with the same voice There are differences, however: Only one computer typically keeps the transmitted information This broadcast medium is being used for point-to-point communication

Ethernet: Computers Talking to Computers When a computer wants to transmit a message: It starts sending signals and also starts listening to see if the message it gets is the one it sent If it is the same message, the computer knows it’s the only computer sending, and it completes the transmission If it isn’t the same message, the computer stops transmitting immediately If the transmission had to stop: Each computer waits a random amount of time and tries to send again They will wait different amounts of time, so one will go first and the other will wait If there is another collision, the process repeats

Getting Connected to the Internet There are two basic methods: Connection via an Internet service provider (ISP) over a WAN Connection provided by an academic institution or an enterprise network over a LAN Most of us use both kinds of connections

Internet Connection via ISP An ISP is a company that sells connections to the Internet The company places a modem at your house that converts the bits a computer outputs into a form that is compatible with the carrier The modem connects to the Internet via the Internet gateway Digital subscriber line (DSL or ADSL) and cable (TV) are two common providers Smartphones also have modems for connecting to wireless networks

Figure 3.6 Connecting to the Internet: LANs, WANS, and ISPs Schematic diagram of connecting to the Internet. In (a), an ISPs modem converts the computers bits to signals the carrier’s technology (phone lines, cable, microwave, etc.) can use; their servers connect to the Internet gateway. In (b), on campus (or at another enterprise), the local network’s server connects directly to the Internet gateway.

What About Wireless? Wireless, or Wi-Fi, is a variation of a LAN connection Referred to by its protocol name: 802.11 Uses a router, which is: Physically connected to an ISP’s modem, and also connected to the Internet Capable of broadcasting and receiving signals, usually radio frequency (RF) signals

Figure 3.7 Wi-Fi and the Internet Standard Wi-Fi network configuration. A wireless router is connected via the modem to the ISPs Internet modem; laptops and other wireless-enabled devices connect by radio signals to the router.

1.5 Once You’re Connected: The World Wide Web Some computers connected to the Internet are web servers Web servers are computers programmed to send files to browsers running on other computers connected to the Internet. These files: Enhance the web page (like images or animations) Help with other web services (like playing audio or video) Web servers and their files make up the World Wide Web (WWW)

Requesting a Web Page Requesting a web page means your browser is a client asking for a file from a web server To see a web page, the request is usually over HTTP/HTTPS (HyperText Transfer Protocol) The page or file requested is a part of the URL (Universal Resource Locator)

Web Pages, Files, and URLs http://homes.cs.washington.edu/~snyder/index.html The example URL has three main parts: Protocol (http): tells the computers how to fetch the file Server computer’s name (homes.cs.washington.edu): can also be the server’s name in the domain hierarchy Page’s pathname (homes/~snyder.index.html): tells the server which file (page) is requested and where to find it

URLs: What Could Go Wrong? An incorrect host name will cause DNS to find the wrong server, or return an error An error in the path will cause the web server to get the wrong page or return an error Note: administrators may arrange that a wrong URL redirects to a corrected one, such as a frequently misspelled word twittr.com could redirect to twitter.com

1.6 Web Pages are Files Servers do not store web pages in the form seen on our screens Pages are stored as a description of how they should appear on the screen. The browser receives the description/source file and creates the web page that is described

Figure 3.11 A Web Page and Its File A web page and the HTML source that produced it. Notice that an additional image file, alto.jpg, is also required to display the page.

Websites and File Structure The file structure forms the directory hierarchy Think of the hierarchy as a tree Folders are the branch points Named collections of files or other folders (or both) Also called directories Files are the leaves Navigation through the hierarchy is ether down or up Down in the hierarchy means into subfolders (towards the leaves) Up in the hierarchy means into folders (toward the root)

Figure 3.12 Directory Hierarchy The hierarchy of this book, highlighting the path to this figure. Downward-pointing triangles are expanded, right-pointing triangles are not.

File Structures and URLs Part of the directory hierarchy is shown in the pathnames of this URL: https://www.nps.gov/yell/learn/photosmultimedia/ The page is specified by the pathname that tells the computer how to navigate through the directory hierarchy to the file Each time there is a slash (/), we move into a subfolder or to the file, or down in the hierarchy

The index.html file exists only if it was created Finding the File Often, the last item in the sequence is a file name, but this is not always necessary or true When a URL ends in a slash, the last item is a folder name, and the server delivers a particular file from it, usually index.htm or index.html The index.html file exists only if it was created

Summary Discussed basic types of communication: point- to-point, multicast, broadcast, synchronous, and asynchronous Developed an understanding of networking, including IP addresses, domains, IP packets, IP protocol, WANS and LANS, Ethernet protocol, ISPs, enterprise networks, and wireless networks Clarified the difference between the Internet and the World Wide Web Looked closely at file hierarchies