How The Internet Works

1. How The Internet Works By PresenterMedia.com

2. Addresses • Two computers on the Internet can only communicate with each other if they are able to distinguish each other from all the computers on the Internet. • In order to do that each computer must have an address. • This address is called an Internet Protocol address or IP address. • The IPV4 address consists of 4 numbers, from 0 to 255 dotted together, such as 134.198.168.8. • Each of these numbers is called an octet because it can be represented by a binary number of 8 bits. (This statement will probably mean nothing to you now, but should make perfect sense in a few weeks.) • Since each of these numbers is between 0 and 255 inclusive, the total number of possible IP addresses is about 4.2 billion.

3. Addresses (continued) • While 4.2 billion seems like a lot, it’s less than the number of people on earth. This implies that each person on earth would not be able to have an IP address. • Since many of us have more than one device that is connected to the Internet (smart phone, computer, tablet, etc) we will soon run out of IP addresses to use. • IPV4 uses 32 bits to store an address. • For this reason a new system IPV6 has been implemented and is currently in use (although it is not used much at this moment). • IPV6 uses 128 bits to store its addresses which yields approximately 2.5x10^39 times as many addresses.

4. Domain Names • IP addresses are hard for people to remember, so to make it simple for us, an alternate form of addressing is domain names. • Domain names are “words” dotted together such as www.cil.cs.scranton.edu. Each domain name is associated with an IP address. It is possible to associate more than one domain name with the same IP address. • It is also possible to associate NO domain name with an IP address. • There is no practical limit to the number of domain names possible.

5. Domain Names (continued) • Computers always communicate with each other using IP addresses. • People usually communicate using domain names.

6. IP Addresses and Domain Names • IP addresses and domain names are assigned by IANA (Internet Assigned Numbers Authority) internationally via 5 regional organizations. • 134.198.x.x belongs to the U. • The University then divvies up the remaining parts. They have given the CS department 168 to use and we have given 134.198.168.x to various systems in our department. • 134.198.168.1 is www.cs.scranton.edu • 134.198.168.8 is www.cil.cs.scranton.edu • Any address that begins 134.198 is associated with scranton.edu

7. Top Level Domains • Top Level domains are the last part of the name and can denote generic organizations or geographic entities. • The edu is the top level domain for x.x.scranton.edu • The com is the top level domain for amazon.com • The us is the top level domain for www.state.pa.us • Common organizational suffixes are .edu, .com, .mil, .gov, .net, .org • Common geographic suffixes are .us, .uk, .de, .au, .ca • Top level domains must be approved by ICANN, e.g. I could not request the domain sidbury.edu because I am not an educational organization.

8. More About Addresses • When a computer is not connected to the Internet it has NO IP address. • A computer may have different IP addresses at different times. • When my notebook computer is connected to the network in this classroom it will be assigned an address by the University • When I connect in my office it will also be assigned an address. • When I take it home, my local home network will assign it an address • All of these addresses will be different and may very well be different from one day to the next.

9. MAC Addresses Networking hardware also has addresses, Media Access Control addresses or MAC addresses. (note MAC denotes this address and mac denotes the brand of computer) Each network hardware device has a MAC address so, for example, if your computer has both a wired connection and a wireless connection then each of them will have a MAC address (and they will be different). “All MAC addresses are unique.” Since MAC addresses are 48 bits long there is a possiblilty of 280 trillion different addresses [The current world population is about 7 billion]

10. Going to the Correct Web Site • The how the internet works Web page is located at the URL http://www.cs.scranton.edu/~cil102/networking_internet.html so when you type this address into the address bar of your browser and press enter that page appears in your browser. • That address has four parts: • http:// -- protocol • www.cs.scranton.edu/ -- domain name of web server • ~cil102 – folder (on the server) containing the page • networking_internet.html – the page itself

11. Going to the Correct Web Site (continued) • networking_internet.html is written in a language called HTML (HyperText Markup Language) which will be rendered into what you see when you go to the page. • Since computers communicate via IP addresses, your browser will contact a local domain name server (DNS) which will eventually tell your browser that the correct IP address is 134.198.168.1 • Your browser will then contact the server at the appropriate IP address and request the page be sent

12. Dynamic and Static IP addresses • A static IP address is permanent. For example the IP address of the web server for the computing sciences department is 134.198.168.1 and will not change (unless the network is reconfigured). • A dynamic IP address is temporary. Dynamic IP addresses are not assigned until a computer is actually connected to the Internet. Why? • Dynamic addresses are assigned by a DHCP (dynamic host configuration protocol) server. Explain

13. How Browsers Work • Web pages are written in a web programming language such as HTML (or one of its derivatives like XHTML). This page is transferred to your browser using a protocol designed to transmit web pages, such as HTTP. The browser will then render the programmed page and it will appear in a browser window (the way it normally looks). The Web site creation lab should make this clear.

14. Interactive Web Pages • Some pages are created with the ability to interact with the reader. For example, my home page has tabs that cause menus to drop down whenever you move your mouse pointer over them. This is accomplished with scripts. • There are two types of scripts: scripts for client side processing and scripts for server side processing. • Client side scripts run on the browser. Server side scripts run on the server.

15. Interactive Web Pages • Client side scripts can be dangerous since they are running on the browser (typically your computer) and could do malicious things. For this reason, some people turn scripts off on their browser. However, my web site (and other more famous sites such as youtube) will not work if scripts are turned off. With the firefox browser, the user has the ability to turn scripts off on some sites while allowing them on other sites. • Server side scripts are run on the server and could be slower since data has to travel to the server before the script can be run. Filling out forms such as registering on a web site or purchasing things online use server side scripts.

16. Choosing a Browser • If you use MS Windows, you already have Internet Explorer. Unfortunately, too many Web sites assume that the visitor has IE so they might not render correctly on other browsers. On the other hand, many malware exploits are designed specifically for IE • Safari is the standard browser under Macintosh OS X. A version is also available for MS Windows (but I see no good reason to use it on windows unless you are a Web site developer). • Firefox is a multi-platform browser with versions available for Windows, OS X, several versions of Linux, BSD, and probably other systems as well. • It has been my experience that if one browser has a good feature then most of its competitors will have that feature shortly.