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Lecture 5

Week 5

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Computer Science
Computer Science 1033A/B
Pamela Glatt

COMP SCI – WEEK 5 Warm up questions • GIF uses a lossy compression ◦ FALSE • 8-bit indexed colour uses 256 colours? ◦ TRUE • Vector images look good even if you re-size them to make them bigger ◦ TRUE Computer network • Network ◦ a group of interconnected computers (could be connected with wires, wirelessly, satellites) • let's look at some ways to configure a network and think about the pros and cons of each configuration • assume we have a map of the United States.....good/bad about certain network layouts?? The Internet • the internet is a global system of interconnected computer networks that use the standardized Internet Protocol Suite (TCP/IP) to serve billions of users worldwide • a network of networks • the internet is hardware, not software! • The WWW is software that runs on the internet How does the Internet work? • Uses TCP/IP • a standard protocol (way of communicating) • the ideas behind this protocol were funded by theAdvanced Research Projects Agency (ARPA) of the US Department of Defence (DoD)--around 1969 • thus the Internet was originally called theARPANET • opposite of your home telephone where you get a direct line that only you and the person you are talking to can use • no direct line at the outset of the message! If a communication line is broken, another line is tried • imagine that I had written a manuscript, printed it but I had NOT stapled it together. I have to get it from our classroom to my publisher in Toronto at Bloor and Yonge. I could: ◦ Idea 1: Give the whole manuscript to one of you and tell you at the beginning to take the whole manuscript, and drive down to Bloor and Yonge in Toronto—and stop all traffic on these roads while you do this This is how a phone line works! ▪ Called Circuit Switching ◦ Idea 2: ???? • each page in the manuscript is similar to a packet • packet: a small group of bytes consisting of a header (tells where it is going: destination, and where it came from: source) and the body. (often 64 bytes for header and 512 bytes for body) • protocol: rules for a format and transmission of data TCP • does a few things ◦ at the sending end: ▪ takes a large chunk of data (such as a web page, email message, etc) and breaks it into small packets ▪ sends the packets out on to the internet ◦ at the receiving end: ▪ detects lost packets, packets with errors because of network congestion, traffic load balancing, or other unpredictable network behaviour, and requests the packet to be resent from the source ▪ rearranges and reassembles the packets back into the web page, email message, etc on the receiving machine IP • like a GPS • picks a route for a packet, stopping at routers which pick the next best machine/network to send the package to • if a communication line is down or broken, sends the package back to TCP and TCP sends it again to try a different route • needs to be able to identify all the machines on the internet, thus each machine has it's own unique address • uses IPadresses IP address • just like your home address • each machine has its own address, called an IP address • consists of 4 numbers with dots between them • each number ranges from 0 to 255 • EXCEPT: IP addresses are NOT geographical • ways to represent it ◦ is always 32 bits How to send the message • TCP breaks web pages into packets of bytes • TCP figures out IP address of where it wants to send the packets (destination) • TCP figures out IP address of where the packet is coming from (source) • sends off each packet to first machine (IP address) on the route (does not pre-plan route) • packet stops at first machine, likely a router; then the router sends it to the next machine on the journey (IP address) and so on until it gets to the final IP address (destination) • called Packet Switching Domain names • in 1973, IP address became the standardized way to identify machines on the internet • in 1984, University of Wisconsin came up with a name server, that maps a name to an IP address • in 1985, Domain Name System is established and the initial top level domain names are introduced • in 1990 internet moves beyond the world of the government and universities and into the commercial society • up until 1995, you didn't have to pay for your domain name, 1995-1998 you paid the NSF (National Science Foundation) $100 US dollars for a 2 year registration for a domain name • in 1998 the assignment of domain name is opened up to private companies to encourage competition How does a domain name work? • every machine on the internet gets and IP address • a DNS (Domain Name System) maps the domain name to the correct IP address • in most cases there is a one to one mapping between an IP address and a domain name • sometimes one IP address might map to more than one domain name • sometimes one domain name might map to more than one IP address More about domain names • domain names identify machines on the internet, for example a web server machine • web server contains all the web pages for a company or individual • web pages are stored on the web server machine in folders or directories • web pages are just files, usually with the extension .htm, for example: myhomepage.html or prices.html URL • a URL (established by Tim Berners Lee in 1990) points at a web page on the internet Domain name systems (DNS) • a domain name system/server (DNS) maps the domain name to the IP address • like a big phone book of Domain names and IP address • break down the domain name: ◦ ◦ www – world wide web: not a part of the domain name ◦ csd – third level domain, also a sub domain: csd is a sub domain of the domain ◦ uwo – second level domain ◦ ca – top level domain (TLD): rules exist for what you can pick, only certain combinations of letter have been established as allowable top level domains ◦ things to note: ▪ the domain is ▪ csd is a sub domain of ▪ www is not part of the domain name Sub domains • used to organize your web server (just like folders and directories organize your computer) Rules for domain names • each item between a dot is called a level • you can have a maximum of 127 levels (thus the top level domain is 1 level and the second level is 1 level, that leaves room for 125 sub domains) • each level can be up to 63 characters long • the entire domain name (including sub domains) can not be more than 255 characters • must use one of the approved TLDs • each level must consist of letters, digits, and hyphens • each level cannot start with a hyphen or end with a hyphen • each level must not contain a space • domain names are case insensitive Top level domain names • an international int
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