CITS1231 Web Technologies. Client, Server, the Internet, and the Web

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CITS1231 Web Technologies

Client, Server, the Internet, and the Web


Topic Outline

• How does the Internet work

– Client Server Architecture – Connections

– Communications Protocols – Addressing

– Routing

• One of the important Internet Applications: WWW



What are the main components here?

Internet Client


Returned Web Pages


What does a client do?

• Once wired, the user accesses the Web via software, called a browser (e.g. Firefox or Internet Explorer).

• Browsers locate and display information from the Web.

• Communication is by an agreed transmission language or protocol, eg. HTTP (HyperText Transfer Protocol).

• The user requests a Web page through the browser, which communicates this to the server.

• The browser waits for the Web page to be delivered, typically a text file containing HTML instructions.

• The intricate graphics and formatting results from the browser rendering that page in the format defined in the file.



What does a server do?

• The server’s job is somewhat easier.

• The server is software running on a computer, and it responds to client requests for Web pages.

• The Web pages exist on its local file system.

• The server retrieves and then transmits the files to the client.


What does the “cloud” do?

• A network is a structure linking computers together for the purpose of sharing resources such as printers and files

• Users typically access a network through a computer called a host or node

• A computer that makes a service available to a network is called a server

• A computer or other device that requests services from a server is called a client

• One of the most common network structures is the client- server architecture



The Largest Inter-connected network – the Internet

• If the computers that make up a network are close together (within a single department or building), then the network is referred to as a local area network (LAN).

• A network that covers a wide area, such as several buildings or cities, is called a wide area network (WAN). The largest WAN in existence is the Internet.

• The Internet was called ARPANET, created by the Advanced Research Projects Agency (ARPA) and the U.S. Department of Defense for scientific and military communications.

• In 1969, it consists of four network nodes, connected by a phone line.


Physical Structure of the Internet

• Today the Internet has grown to include hundreds of millions of interconnected computers, cell phones, PDAs, televisions, and networks

• The physical structure of the Internet uses fiber-optic cables, satellites, phone lines, and other telecommunications media



Structure of the Internet


In summary, the Intenet is

• A network connects computers so they can communicate, exchange information, and share resources.

• The Internet is an infrastructure, in particular a global computer network, supporting data transmission.

• The Internet is a network of interconnected networks. If part of its infrastructure is destroyed, data can still flow (in

principle) through the remaining networks.

• The Internet uses high-speed data lines (backbones) to carry data. Smaller networks connect to the backbone, enabling any user on any network to exchange data with any other user.



How the Internet works


• Addressing Schemes

• Domains and Sub-domains

• Routing Traffic Across the Internet


How the Internet works - Protocols

• Network communication is made possible only if computers

“speak” a common language. The rules and procedures for controlling timing and data format are the protocols and they,

– signal another computer requesting communication. (client) – identify the requesting computer. (server)

– transmit messages in blocks. (server)

– retransmit if messages fail to arrive. (server) – detect errors and recovers. (client)

– signal transmission is complete. (client) – terminate the connection. (server)



How the Internet Works – TCP/IP

• Every computer and network on the Internet uses the same

protocols - the Transmission Control Protocol/Internet Protocol, or TCP/IP.

• No matter what type of computer system you connect to the

Internet, if it uses TCP/IP, it can exchange data with any other type of computer.

• TCP/IP was developed to tolerate unreliable sub-networks and the protocol guarantees proper transmission of data, since the physical network can’t.

• For transmission not needing guarantees (even unreliable networks are very reliable) one can use User Datagram Protocol (UDP). Data transmitted by UDP arrive faster, with none of the error detection or correction overheads that are in TCP/IP.


0101010010 0010101110 1110010110 0010010000 0010010101


0101010010 0010101110 1110010110 0010010000 0010010101


0101010010 0010101110 1110010110 0010010000 0010010101


0101010010 0010101110 1110010110 0010010000 0010010101




0101010010 0010101110 1110010110





0101010010 0010101110 1110010110



How the Internet works - Addressing

• In order to communicate across the Internet, a computer must have a unique address.

• Every computer on the Internet has a unique numeric identifier, called an Internet Protocol (IP) address.

• Each IP address has four parts – each part a number between 0 and 255.

An IP address can have up to 12 digits and might look like:

• Our departmental email computer (the mail server) has the IP address,, and no other machine in the world has this IP number.

Otherwise you would never get your email.

• Since we do not think in terms of long integers, host machines can have a symbolic name(s). For our machine, the symbolic name is, consisting of the host name (mailhost) followed



How the Internet works - Domains

• So in addition to an IP address, most Internet hosts or servers have a domain name address, using words.

• A domain name identifies the type of institution that owns the computer. An Internet server owned by IBM might have the domain name

• The domain name is itself made up of name levels so that, au is Australia, edu covers educational sites (within Australia), uwa is The University of Western Australia.

• Some enterprises have multiple servers, and identify them with subdomains, such as


Internet Domains



How the Internet works – Domain Name Server

• As far as the Internet is concerned the symbolic machine names are “eye candy” for human consumption.

• IP addresses are necessary for computer communication.

• The IP addresses are provided by Domain Name Server (DNS) computers that map symbolic names to their IP.

• Computers on a network are designated DNS machines and they are responsible for providing the IP mapping and for the upkeep of the database as new machines and IP are added to the Internet.


How the Internet works - Alias

• A host machine must have one, single and unique IP address, but it can have any number of symbolic names. So the

machine www is also known as and, which are redirected to

• As an exercise visit

– –




Routing Traffic Across the Internet

• Most computers connect to the Internet via a smaller network that is connected to the Internet backbone.

• The Internet includes thousands of host computers (servers), which provide data and services as requested by client


• When you use the Internet, your client requests data from a host system. The request and data are broken into packets and travel across multiple networks before being reassembled at their destination.



An Example

• Say I have a 300 Kb file to transmit to Sydney.

• The software sending that file first splits it into reasonable size packets, say 15Kb each.

• Each packet has a header containing essential information like, destination IP address, source IP address, size of the

transmission and packet position in the original file.

• But what path through the network does it take to get to its destination?


Dynamic routing

• For robust networks, a transmission protocol must find new routes to a destination as preferred routes fail.

• This is achieved by dynamic routing, where the routes are

selected at the time of transmission, after considering current network conditions.

• Dynamic routing requires a network architecture devoid of

critical sites, whose failure will bring down the entire network.

That is, the network cannot be hierarchical.



Dynamic routing continued

• The Internet was designed on a lattice or graph, where there are a large number of widely distributed paths.

• The hosts performing routing duties are called routers, of which there are thousands on the Internet.

• Back to our example

– The message packets need not travel in order, or follow the

same path. The packets are sent to a router, which based on the destination address decides where to send the packets (usually to another router).

– At every point in time, the current router decides where to send the packets next. The packets are transmitted with the

intermediate destinations assigned as they go.


Example Continued

– Note that if some packets are sent to a particular router, it may be possible that the subsequent packets are sent to a different router, if the original router becomes unavailable.

– Eventually all packets arrive at their destination, possibly out of order because they have taken different journeys.

– If packets fail to arrive or arrive corrupted, the destination host transmits a message to the original host to re-send the

particular packets.

– Once all packets are gathered the original message can be reconstructed from the ordering contained in the packet headers.



Popular applications of the Internet

• Many users confuse the Internet with applications that work over the Internet. There are many such applications but the six listed below are amongst the most popular.

– E-Mail – News – Telnet

– File Transfer Protocol (FTP) – Internet Relay Chat (IRC) – The World Wide Web


The History of the Word Wide Web

• Timothy Berners-Lee and other researchers at the CERN nuclear research facility near Geneva, Switzerland laid the foundations for the World Wide Web, or the Web, in 1989

• They developed a system of interconnected hypertext

documents that allowed their users to easily navigate from one topic to another

• Hypertext is a method of organizing information that gives the reader control over the order in which the information is presented



Hypertext Documents

• When you read a book, you follow a linear progression, reading one page after another

• With hypertext, you progress through pages in whatever way is best suited to you and your objectives

• Hypertext lets you skip from one topic to another


Linear versus hypertext documents



Hypertext Documents

• The key to hypertext is the use of hyperlinks (or links) which are the elements in a hypertext document that allow you to jump from one topic to another

• A link may point to another section of the same document, or to another document entirely

• A link can open a document on your computer, or through the Internet, a document on a computer anywhere in the world

• An entire collection of linked documents is referred to as a Web site

• The hypertext documents within a Web site are known as Web pages

• Individual pages can contain text, audio, video, and even programs that can be run remotely


In Summary, the World Wide Web is

• A distributed hypertext system

• Defined by a set of common communication protocols

• Accessible from a variety of platforms

• Resources on the Web about WWW and its protocols:



Pondering Moment

• What do I do when I surf the Web?

• If I were to create a website, what would be the purpose?

– provide a service – sell a product

– present information on a topic – make an announcement

– inform people about yourself

– create a forum that addresses a certain issue – deliver news on a particular subject




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