Internet Basics

What is the Internet?

What is the Internet? The Internet is a network of networks of computers.

Parts of the Internet What are some parts of the Internet?

Parts of the Internet World Wide Web Telnet Email Ftp

What is the World Wide Web?

What is the World Wide Web? The World Wide Web is a hyperlinked network of documents and other resources found on the computers of the Internet.

Locating resources In order for the WWW to be useful, we need a way of referencing all the resourses available.

URL What does URL stand for?

URL What does URL stand for? Uniform Resource Locator

URL What does a URL do?

URL What does a URL do? A URL allows every resource (e.g. HTML page, image, sound clip etc.) on the WWW to have a unique address.

Parts of a URL http://www.someaddress.com:8080/files/intro.html

Parts of a URL The protocol gives the method of communication to be used. http is most common, but you may see ftp as well.

Parts of a URL The domain name is the name of the computer that has the resource you want. This computer is often called the host.

Domain names Domain names are broken down into different levels. E.g. www.someaddress.com The top level domain name is com The second level domain name is someaddress The third level domain name is www

Domain names What are some top level domain names? How many levels do there have to be? Does the lowest level domain have to be www? Are domain names case sensitive? What is the highest price paid for a domain name?

Domain names Host machines actually have IP (internet protocol) addresses, not domain names. IP addresses have the form 255.255.255.255 A series of Domain Name Servers keep lists which map domain names to IP addresses.

Domain names Why bother with domain names? Why not just use IP addresses and save looking them up? How can we easily find an IP address for a given domain name and vice versa?

Parts of a URL The port specifies the port number that the server is listening to for requests. Port number is optional If not given, the default of port 80 is used.

Port number Why use different port numbers?

Parts of a URL The exact path to the desired resource follows the domain name (and port number if given).

Directory and resource path This is a description of the file structure on the host system. If the host is Unix or Linux, this part of the URL is case-sensitive. To avoid problems it is best to use lower case for all resource paths, on all systems.

Directory and resource path Different levels of a path are always separated by forward slashes, regardless of the host system. If multiple users are hosted on a system, the first part of the path will be ~user

Directory and resource path The last part of the path is the actual resource desired, usually an HTML page. The extension for HTML files can be .htm or .html (recommended) If no resource is given, the server will try to display a page named index.html. If not found, it may display a directory listing, if permitted.

Directory and resource path A fragment identifier can be used to specify a given part of an HTML page. E.g. the URL http://www.someaddress.com/files/intro.html#part3 will show the page intro.html, starting with the section labelled part3. This is useful for long documents.

Paths Since graphics are not embedded in web pages, the location of the graphic is indicated to the web browser with a path in HTML.

Relative vs. absolute URLs In Unix terms what is the difference? In URL terms what is the difference? When are relative URLs used? Why? What problems can there be with use of relative URLs?

Packet switching Packet switching is one of the key concepts of the Internet. This involves the use of two separate concepts, packets and switching.

Packets A message is broken up into small pieces called packets, usually about 1 kb in size. Each packet recieves a header containing the destination IP address, the sender’s IP address, the total number of packets that make up a message, and the sequence number of that packet.

Switching Since the packets are individually addressed, and numbered for sequence, they can be sent and received in any order. This means that packets can be switched to different routes to get to the destinations, according to network traffic.

Request/Response The HTTP protocol is set up to work in terms of requests and responses.

Request/Response In a typical WWW example, you type in a URL in your browser’s location window, and press enter. Your browser then sends a message (request) to a web server, asking for a given HTML page. The web server sends back the page, or a reason it can’t comply (response).

A Typical Internet Request You type in a URL in your web browser (http://www.test.com) The web browser needs to know the IP address that is assigned to this URL, so it makes a request to a Domain Name Server (DNS). The request gets passed along from one name server to the next, until the address is found, or the request times out.

A Typical Internet Request Once the browser has the IP address (123.456.789.123), it can prepare the HTTP request packets and send them to the server. The message is divided into packets, which can be addressed with the source and destination IP addresses.

A Typical Internet Request The message packets are then sent on their way. From a dial-up connection, the first step is the ISP. The ISPs router looks at the destination address, and if it can’t deliver the message, it passes it on to another router, etc. until the message reaches its destination.

A Typical Internet Request The message reaches its destination site, usually a web server, which processes the request. It then gets the requested page, if available, and prepares a reply message, including the requested information, and then divides it into packets and sends it back to the originator of the request.

Standards on the Internet/WWW Technical standards related to the Internet (e.g. TCP/IP) are published as Requests for Comment (RFCs). These are very technical, detailed descriptions of the technology behind the Internet. URL - www.ietf.org

Standards on the Internet/WWW Standards related to the World Wide Web (e.g. HTML) are set by the World Wide Web Consortium (W3C) URL – www.w3.org Standards released by the W3C are not necessarily implemented completely or in the same way by different web browsers.

Searching the web Search engines are necessary to even attempt to catalogue the estimated 270,000,000 web sites, with an estimated 7,000,000 being added daily. What are some popular search engines? How do they work?

IP addresses The current method of IP addressing uses 32 bit addresses, and we are quickly running out of addresses Work is underway to introduce IPv6 or IPng, which uses 128 bit addressing.

IP addresses This will provide 340,282,366,920,938,463,463,374,607,431,768,211,456 addresses, which could give up to 3,911,873,538,269,506,102 addresses per square metre of the Earth’s surface.

Review What is the Internet? What is the WWW? Is email a part of the Internet? What are the parts of a URL? What are the parts of a domain name?

Review How does packet switching make Internet communication more reliable? What organization makes decisions about HTML standards? Are HTML standards always followed by the different browsers?

Web browsers A Web browser contains the basic software you need in order to find, retrieve, view, and send information over the Internet. This includes software that lets you: Send and receive electronic-mail (or e-mail) messages worldwide nearly instantaneously. Read messages from newsgroups (or forums) about thousands of topics in which users share information and opinions. Browse the World Wide Web (or Web) where you can find a rich variety of text, graphics, and interactive information.

FTP/TELNET TELNET: The Internet allows computers to converse with each other over networks. A telnet program allows us to log into a distant computer almost as if we were actually sitting physically at that computer. FTP: File Transfer Protocol allows us to transfer files between two different computers on the Internet.