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Internet. COMP 101 Spring Semester, 2006 ( Last Modified: 2006/02/09 ). Local area network (LAN). A computer may be connected to a number of expensive devices: e.g., color laser printer, large disk of useful data, etc.

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COMP 101

Spring Semester, 2006

(Last Modified: 2006/02/09)

Local area network lan

Local area network (LAN)

  • A computer may be connected to a number of expensive devices: e.g., color laser printer, large disk of useful data, etc.

  • A LAN is used to connect a number of computers so that these devices can be shared by these computers.

  • LAN operates over a limited distance.

Wide area network wan

Wide Area Network (WAN)

  • It is desirable to enable sharing among computers over a wide geographic area.

  • Each LAN in a WAN has a dedicated computer attached to the transmission line (the “gateway” or “router”).

  • This computer delivers incoming messages to the designated computers on the LAN and collects outgoing messages to send off.

The objective

The objective

  • Make the network transparent to the user

Communication protocols

Communication protocols

  • For a computer to send a message to another, they must at least agree on:

    • Cable connector design

    • Voltage of electrical signals

    • Frequency of signal modulation

    • Interpretation of data being transmitted

The internet

The Internet

  • An “open” network using the TCP/IP protocol to “interconnect” existing networks

  • Initial development funded by US Defense Department around 1969

  • Mature technology being used in private networks as well

Internet protocol ip

Internet protocol (IP)

  • Messages on the Internet are transmitted in “packets”.

  • All computers on the network are uniquely identified by an IP address.

  • Identical IP software on the computers can interpret the address on each packet to decide which computer to send the packet on.



  • Routers are dedicated computers on the Internet that run identical IP software.

  • A router interprets the IP address on an incoming packet and uses an internal routing table to forward the packet to another router connected to it.

Transmission control protocol tcp

Transmission control protocol (TCP)

  • A message may be divided into a number of segments before transmission.

  • Each segment is “wrapped” inside an IP packet.

  • Packets may be lost due to equipment/network failure or congestion.

  • TCP software checks for lost packets and requests retransmission, and removes duplicate packets.

  • TCP software assembles packets in the original order at destination.

Other protocols discussed in comp101

Other protocols discussed in COMP101

Domain name system dns

Domain name system (DNS)

  • Service to translate the understandable addresses into IP addresses

  • Not case-sensitive

  • Examples of top-level domains:

    .com, .edu, .gov, .org, .hk, .cn

  • To find domain information from IP addresses: use

International domain names

International domain names

  • You may register with CNNIC:

    • 香港工程師.中國

    • 香港工程師學會.網絡

    • 香港工程師學會.中國

    • 香港工程師學會.公司

    • 香港總彩公司.com

    • 香港長江實業.com

    • 香港匯豐銀行.com

    • 香港民意網.com

  • No live website yet 



  • A private network using TCP/IP



  • A software filter to prevent harmful messages from reaching an Intranet

Internet under stress on 9 11

Internet under stress on 9-11

  • No disruption outside of the attack zones

  • Websites of major news services not reachable due to overload

  • E-mails remain effective communication channel after phones failed

Future internet stress

Future Internet stress

  • Lack of unique IP addresses

    • Solution: moving to IPv6

  • Viruses and worms bring down connected computers

    • Solution: more effective firewall/filter

  • SPAM messages clog up bandwidth

    • Solution: more effective SPAM filter

Ip header format

IP header format

Tcp segment format


15 16


TCP segment format

source port number

destination port number

sequence number

acknowledgement number

header length

reserved (6 bit)







advertised receive window size

TCP checksum

urgent pointer

options (if any) <variable length>

data (if any) <variable length>

Ip addresses

IP addresses

  • 4-byte unique identification of computer

  • The address for the computer “” was

  • The address for the computer “” was

  • Running “ping” in “cmd” window can find the latest IP address

Finding the ip address of a computer

Finding the IP address of a computer

  • Run cmd on Windows machines

  • Login