Transcript Internet and WWW - Computer Science Department

Internet and WWW
CS216
Open System Interconnection (OSI)
OSI
TCP/IP
• The Internet is the communications medium
that uses the TCP/IP protocol. The World Wide
Web (WWW) is a service that uses the
Internet. The Internet had its beginnings
around 1969 with work done by the defense
department to establish a reliable
communications network.
World Wide Web
• The WWW began in Europe, as an effort of the European
Laboratory for Particle Physics (CERN) as a method for
researchers to share results.
• The WWW is an architected way to send and receive web
pages of information. Each user has a browser program.
From the browser you can request web pages. The web
page uses the Hyper Text Markup Language (HTML)
language. The browser gets a file of data containing HTML
keywords and data and other files (for example graphics
files), and displays the web page. The user can send data
back to the sender of the web page, request other web
pages (via its Uniform Resource Locator or a hyperlink).
Web pages are requested by their Uniform Resource
Locator (URL).
URL
• The URL has the format:
– scheme://internet-address/resource
• The scheme describes the protocol used to access the
resource. It is usually http://, the protocol used by the
World Wide Web (WWW).
• The internet address is the node where the resource
(usually a file) is located.
• The resource name follows the slash. This may be just a
file name, or it may contain directory information for
the server to locate it on the node.
• For the WWW, the protocol is hypertext transfer
protocol (http). Some other Internet protocols are:
Other protocols
•
•
•
•
•
news (newsgroups)
ftp (file transfer protocol)
telnet
https
You can create one yourself
Domain names
• Web_page_name.domain name is the symbolic name
of the page (for example www.ibm.com,
www.uky.edu). In the U.S., the highest level domain
names [top level domain], furthest on the right) are:
• com - businesses
• edu - educational institutions
• gov - government organizations
• mil - military organizations
• org - nonprofit organizations
• net - network resources
ICANN
• Other countries have their own domain
names, for example, uk for England. Because
of the overuse of the .com domain name, the
governing body for the Internet the governing
organization for the Internet (Internet
Corporation for Assigned Names and Numbers
– ICANN) has created other domain names
that are not country specific.
DNS
• URLs are a human convenience. All Internet addresses
are (for now) four byte addresses called IP addresses.
They are expressed as four decimal numbers separated
by periods: 128.163.146.153. This is the IP address for
the CS server. Besides URLs, browsers will accept IP
addresses.
• A hyperlink is a shortcut to a URL. A hyperlink to
another web page is highlighted in some manner
(usually a different color and underlined) on the page.
By clicking on the hyperlink, your browser requests
that web page. This is done by HTML tags.
Connecting to the Internet
• In homes, you can connect to a network via a
digital subscription line (DSL), a TV cable
company, or a phone line. DSL and cable
connections are high speed (also called broad
band) connections. The phone line is much
lower speed, but you do not pay any extra
charges over the cost of your phone
connection.
Modem
• The modem (modulator/demodulator) converts the serial
digital bits of information into analog information that can
be sent across the lines. This modulation process is
accomplished by varying the amplitude or frequency of a
carrier tone transmitted on the phone line.
• Standard modems advertise 56,000 bits per second
transmission capability. However because of the errors in
data transmission (phone lines weren’t designed for
transmission of bits of information) causing retransmission
of data, plus the added bits needed for control, error
detection, and framing data into packets, the effective rate
of transmission is perhaps half that.
• DSL overcomes that problem by superimposing higher
frequency signal on low frequency carrier
LAN
• In offices and schools, the medium is usually a
cable, and the connection of workstations
together into a network is called a local area
network (LAN)
LAN
Wide Area Networks
• A network (such as the Internet) that covers a
wide geographical area is called a Wide Area
Network (WAN). Besides the Internet,
businesses or government organizations may
have their own WANs.
WAN
• The computer at each end of a communications link must follow the same
rules. These communications rules are called protocols. The protocol
followed for the Internet is called TCP/IP.
• When accessing a website via URL addresses, DNS is used for conversion
to TCP/IP addresses and a connection is made to the destination.
• User communications programs (like Internet browsers, e-mail programs)
use the operating system communication services. If you know the
operating system interface for TCP/IP, or FTP (the file transfer program)
you can use them directly from the command line, however using the
communications programs (such as a browser) is much easier.
• If you have to develop a web server application to respond to web page
requests, you would have to learn more about the underlying TCP/IP
protocol. Common terms used in a TCP/IP connection are sockets, ports,
and the secure shell.
TCP / IP
• TCP – Transfer Control Protocol
• IP – Internet Protocol
• Key concept: they are two distinct protocols
that coexist
– Each machine (for practical purposes) has one IP
address
– However, it can have many ports (TCP address)
TCP Ports
• Addressing on top of IP so that multiple
communication channels can exist simultaneously
• Common port assignments:
–
–
–
–
Port 23 is TELNET
Port 25 is SMTP
Port 80 is HTTP
Port 8080 is also used for HTTP sometimes so that two
servers can “live” together on the same machne