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THE INTERNET

The Internet

• The Internet: An internet that spans the world– Original goal was to develop a means of connecting

networks that would not be disrupted by local disasters.

The Internet • An information system• Communication tool• A large network of computer systems

world wide• can be explored by using a Web Browser

• Services available through the Internet:• Email (Discussion groups , Chat Rooms, etc.)• Information search (including research)• Education (Online courses; virtual colleges)• Entertainment, News, and more…...

Internet Architecture

• Internet Service Provider (ISP)• Access ISP: Provides connectivity to the Internet

– Traditional telephone (dial up connection)– Cable connections– Wireless

Internet Composition

A network protocol Is the set of very detailed rules, sequences, message formats, and procedures that computer systems use and understand when exchanging data with each other.

Examples of Protocols– Transmission Control Protocol (TCP)– User Datagram Protocol(UDP)– File Transfer Protocol (FTP)– Hypertext Transfer Protocol (HTTP)– Simple Mail Transfer Protocol (SMTP)

Transmission Control Protocol (TCP/IP)

• It is connection oriented.• It establish connection before data transfer • It divide the whole message in small parts known as

packets• It sends packets in order and it also receive in order

in destination

User Datagram Protocol(UDP)

• It is not connection oriented.• It does not establish connection before data transfer • It divide the whole message in small parts known as

packets.• It sends packets to the available path on that

particular movement. • The packets are not received in order and it required

a addition mechanism for ordering the packets on destination.

File transfer protocol

• Is a network protocol used for transfer data from one computer to another through a network such as Internet (is commonly used for copying files to and from other computers)

• FTP is a two-way system - it can be used to copy or move files from a server to a client computer as well as upload or transfer files from a client to a server

• FTP systems generally encode and transmit their data in binary sets which allow for faster data transfer

What is an IP Address?

• An IP address is a unique global address for a network interface– An IP address:

- is a 32 bit long identifier

- encodes a network number (network prefix) and a host number.

• The network prefix identifies a network and the host number identifies a specific host (actually, interface on the network).

Network prefix and host number

network prefix host number

Dotted Decimal Notation

• IP addresses are written in a so-called dotted decimal notation

• Each byte is identified by a decimal number in the range [0..255]

• Example:

1000111110000000 10001001 100100001st Byte

= 128

2nd Byte

= 143

3rd Byte

= 137

4th Byte

= 144

128.143.137.144

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IP Addresses (continued)

• When IP addresses were originally created, they were called classful addresses– That is, each IP address fell into particular class– A particular class address has a unique network address

size and a unique host address size– There are basically five types of IP addresses: Classes A,

B, C, D and E

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IP Addresses (continued)

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IP Addresses (continued)

• When you examine the first decimal value in the dotted decimal notation:– All Class A addresses are in the range 0 - 127– All Class B addresses are in the range 128 - 191– All Class C addresses are in the range 192 – 223– All Class D addresses are in the range 224 – 239– All Class E addresses are in the range 240 - 255

Name servers

DNS• IP addresses are difficult to remember

– [email protected] would be difficult to remember– If JohnDoe’s mail server moves to another machine, then his e-mail address

would not be valid anymore– Something like [email protected] would be appropriate

• Need some mechanisms to translate wuzwuz.ucg.ie to the IP address– To solve this problem DNS was invented

• It is a hierarchical, domain-based naming scheme and a distributed database system for implementing naming scheme.

• Usage:– Map name onto an IP address, an application program calls an library

procedure, called resolver.– The resolver sends packet to a local DNS server which looks up the name and

returns the IP address to the resolver– The resolver returns the IP address to the application, which can establish an

TCP/IP connection with the destination (or send packets)

URL – Uniform Resource Locator

URL – Uniform Resource Locator

Traditional Internet Applications

• Electronic Mail (email)– Domain mail server collects incoming mail and transmits

outgoing mail– Mail server delivers collected incoming mail to clients

via POP3 or IMAP

• File Transfer Protocol (FTP)• WWW (World Wide Web)

World Wide Web (WWW) – a vast collection of information that’s connected like a

web– accessible through the Internet using connections called

hyperlinks– view the WWW resources through a program called a

Web Browser, such as:• Netscape Navigator • Internet Explorer

– Navigate through the WWW by pointing to and clicking on hyperlinks (underlined or boldfaced words,

phrases, icons, or images)– Also called Cyberspace or the Information Superhighway

Architectural Overview

• Browser displays a page on the client machine– Click on a link, the browser sends a message to the abcd.com web server asking it for the

page– When page arrives, it is displayed; if it contains a hyperlink on a page on xyz.com, that is

clicked, then the browser will send a message to xyz.com server and the process continues

HTML – HyperText Markup Language

• (a) HTML source code – Markup language containing explicit commands for formatting

• (b) Formatted page