Dr Michael Rees

School of Information Technology

mrees@bond.edu.au

INFT11/71-132

Web Applications

Internet Concepts

The Internet

© 2009 Michael Rees Web Applications 2

• The Internet is a global, wide area network (WAN) of computers

• Composed of millions of smaller local area networks (LANs) all connected together

• All computers support Internet Protocol (IP)

• Each computer must have a unique IP address: a unique 32 bit value made up of 4 numbers (each 0-255)

• Example IP Address: – Binary:

11101110.00010001.10011111.00000100– Decimal: 238.17.159.4

Internet Protocol

• Transfer data from source device to destination device (computers today but any device in future – the Internet of Things)

• IP source software creates a packet representing the data:– About 1500 characters– Header: source and destination IP addresses, length

of data, and so on– Data itself– Checksum used to test data integrity and correct

transmission

• If destination is on another LAN, packet is sent to a gateway or router that connects one network to another

© 2009 Michael Rees Web Applications 3

Web Applications 4

Packet switched network

© 2009 Michael Rees

• TCP (Transfer Control Protocol) breaks data into packets at the sending end and reassembled at the receiving end

• Packets pass from a source computer, possibly through several intermediate computers to the destination computer

• Computers acting as routers examine the packets and decide the onward path each packet should travel

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Transmission Control Protocol

• TCP overcomes the limitations of IP:– No guarantee of packet delivery (packets

can be dropped)– Communication is one-way (source to

destination)

• TCP adds concept of a connection on top of IP:– Provides guarantee that packets are

delivered– Provide two-way (full duplex)

communication

© 2009 Michael Rees

TCP Handshake

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Source Destination

Can I talk to you?

OK. Can I talk to you?

OK

Here’s a packet

Got it

Here’s a packet

Here’s a resent packet

Got it

Establishconnection {

{

{

Send packetwithacknowledgment

Resend packet ifno (or delayed)acknowledgment

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TCP Ports

• TCP also adds concept of a port

• TCP header contains port number representing an application program on the destination computer

• Some port numbers have standard meanings

• Other port numbers are available first-come-first served to any application

© 2009 Michael Rees

TCP Ports

• Port numbers identify particular services on an Internet host computer

• Examples:– port 25 used for SMTP

(mail transfers)– port 23 used for

TELNET (remote host connection)

– port 80 used for HTTP (web page transfers)

© 2009 Michael Rees Web Applications 8

User Datagram Protocol - UDP

• Like TCP in that:– Builds on IP– Provides port concept

• Unlike TCP in that:– No connection concept– No transmission guarantee

• Advantage of UDP vs. TCP:– Lightweight, so faster for one-time

messages

© 2009 Michael Rees Web Applications 9

Domain Name Service - DNS

• DNS RFC

• DNS is the “phone book” for the Internet– Map between host names and IP addresses– DNS often uses UDP for communication

• Host names– Labels separated by dots:

• www.example.org– Final label is a top-level domain

• Generic: .com, .org, .tv, .name• Country-code: .au, .us, .il

© 2009 Michael Rees 10Web Applications

Domain Name System

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• Allows human-readable domain names to map to IP addresses (try: nslookup www.google.com)

• Used in Universal Resource Locator (URL):

http://www.bond.edu.au/bondit/index.htm

protocol

server

domain

folder

file

www.bond.edu.au is a Fully Qualified Domain Name - FQDN

So, who runs the Internet?

• ISOC (The Internet Society) – Is a non profit central organization for the

groups that are responsible for the standards for internet structure

– http://www.isoc.org

• ICANN (Internet Corporation for Assigned Names and Numbers) – http://www.icann.org

• W3C (World Wide Web Consortium) – develops standards for the web– http://www.w3.org

Web Applications 12© 2009 Michael Rees