The document discusses fundamental concepts relating to how the internet works. It can be summarized as follows: (1) The internet is a global network of interconnected computers that uses internet protocol (IP) to transfer data packets from source devices to destinations; (2) IP packets are routed through networks of routers and gateways to reach their destinations, while protocols like TCP and UDP provide reliable or fast transmission; (3) TCP and UDP use port numbers to identify network applications on devices.

1. INFT11/71-132
Web Applications
Internet Concepts
Dr Michael Rees
School of Information Technology
mrees@bond.edu.au

2. The Internet
• 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
© 2009 Michael Rees Web Applications 2

3. 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

4. Packet switched network
• 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
© 2009 Michael Rees Web Applications 4

5. 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 Web Applications 5

6. TCP Handshake
Can I talk to you?
Establish
connection { OK. Can I talk to you?
OK
Here’s a packet
Send packet
with
acknowledgment { Source
Got it
Destination
Here’s a packet
Resend packet if
no (or delayed)
acknowledgment { Here’s a resent packet
Got it
© 2009 Michael Rees Web Applications 6

7. 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 Web Applications 7

8. 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

9. 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

10. 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 Web Applications 10

11. Domain Name System
• 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
domain file
protocol folder
server
www.bond.edu.au is a Fully Qualified Domain Name - FQDN
© 2009 Michael Rees Web Applications 11

12. 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
© 2009 Michael Rees Web Applications 12