Transcript Introduction - Seneca - School of Information & Communications

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
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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
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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
1. You type in a URL in your web browser
(http://www.test.com)
2. 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
3. Once the browser has the IP address
(123.456.789.123), it can prepare the
HTTP request packets and send them to
the server.
4. The message is divided into packets,
which can be addressed with the source
and destination IP addresses.
A Typical Internet Request
5. The message packets are then sent on their
way.
6. From a dial-up connection, the first step is
the ISP.
7. 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
8. 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
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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.