What is port number

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What is port number?

• OK, you know that in order to connect to
  Internet, each computer must have a unique
  address
• IP address (a 32 bits number)
• but that is not all
• a computer can wear many hats
• it can be a web server, a mail server, a telnet
  server, a ftp server, a proxy server and a
  newsgroup server all at the same time

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What is port number?

• So if a client sends a request to a computer
• IP address is not enough
• also need to specify the kind of service
• Port number
• a 16-bits number (0 - 65535 in decimal)
• telnet port number is 23
• www port number is 80

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• Default port number
• for well-known services such as WWW, if you do
  not specify the port number, the client software
  will use the default port number
• e.g. http//www.cuhk.edu.hk/
• no port number !!
• Default 80 is used
• sometimes you see http//www.xyz.com2000/
• which means please send this request to
  www.xyz.com with port number 2000

4

• Port number is a simple idea
• pretty much like telephone extension
• server company
• IP address company phone number
• port number extension within the company
• IP port number
• let you connect to a program located at the
  specified port number of the server

5

• Use of port number
• Proxy server
• Internet access is slow, can we speed it up?

Bottleneck
client
router
WWW server
Campus LAN fast (100Mbps) inexpensive
Oversea Link slow (3Mbps) expensive
6

• What is a proxy
• a agent (software) that helps you to get the data

Bottleneck
client
proxy
router
WWW server
Big disk cache
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• How proxy can speed up the access
• you request for a HTML file
• but you send the IP packet to the proxy server
  instead of the web server
• the proxy server (in you local LAN) checks
  whether it has the file
• YES! Great, return the file to you, no need to
  access remote web server. Quick
• NO! Too bad. Go to the web server, get the file,
  BUT SAVE a copy at the proxy server, then return
  the file to you
• next time if someone asks for the same file,
  proxy will have it

8

• Proxy saves bandwidth
• because the expensive oversea link can be used
  less
• in CUHK, all external traffic must pass through
  proxy
• therefore you must tell your browser to send all
  requests to a proxy server instead of the web
  server
• How?
• Browser -gt select preference -gt advanced
• specified the name and port number of proxy
• proxy.cuhk.edu.hk at port number 8080

9

• What is HKIX?
• Hong Kong Internet Exchange
• a project in CUHK operated by CSC
• the goal is to interconnect the Internet Access
  Providers (IAPs) in Hong Kong
• Why?
• IAPs usually have their own links to U.S internet
  backbone
• but local (within Hong Kong) can be slow !

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• Before HKIX
• for client at IAP A to access a web server at IAP
  B
• send IP packets to U.S., then from U.S. to IAP B
• return the file from IAP B to U.S, then from U.S.
  to IAP A
• slow
• After HKIX
• IAP A sends packets to IAP B via HKIX
• CUHK is the Internet hub for IAPs and HARNET

12

• Your company is on the Internet, do you feel a
  sense of insecurity?
• Sure
• all your computers are connected to Internet,
  great, but that means all the computers in your
  company can also be reached by the hackers !
• How can you reduce the risk?
• Build a firewall

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• What is a firewall?
• A security measure that separate the computers of
  the company from the outside world

Internet
Firewall
Company LAN
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• Firewall is just a server that has the
  responsibility of protecting the companys
  network from intruders
• Proxy - the simplest firewall !
• Why?
• All outgoing traffic MUST go through the firewall
  (proxy)
• that means if a client wants to access an
  external web server, the client MUST send the
  request to the proxy, NOT to web server
• also ALL incoming traffic MUST go through the
  firewall

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• What is the advantage?
• Because all traffic must go through the proxy,
  you can concentrate all your defense at that
  point (the firewall server)
• Without the firewall, hackers can attack any
  computers in your LAN, it is difficult to protect
  so many computers
• so port number is useful in designing proxy
• proxy is useful in building firewall

16

• DNS - domain name service
• Use
• to translate a name into IP address
• example
• you type http//www.cuhk.edu.com/ in your browser
• your browser must first find the IP address of
  www.cuhk.edu.com
• like given my name M.Chang, you need to look up
  the telephone directory and get my phone number
  before you can contact me

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• Where can you find the IP address?
• Answer in a computer called name server
• What is the name of the name server?
• .
• What is the IP address of . ?
• Already build-in in your browser
• suppose you want to find the IP address of
  www.cuhk.edu.hk, you send a message to .

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• Does . know the IP address of
  www.cuhk.edu.hk?
• NO !!
• It only knows the address of a name server called
  hk
• so . sends the enquiry to hk

19

• Does hk know the IP address of
  www.cuhk.edu.hk?
• NO !!
• It only knows the address of a name server called
  edu.hk
• so . sends the enquiry to edu.hk

20

• Does edu.hk know the IP address of
  www.cuhk.edu.hk?
• NO !!
• It only knows the address of a name server called
  cuhk.edu.hk
• so . sends the enquiry to cuhk.edu.hk

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• Does cuhk.edu.hk know the IP address of
  www.cuhk.edu.hk?
• YES !!
• This time cuhk.edu.hk finds that
  www.cuhk.edu.hk is a name registered in its own
  directory, so it returns the IP address to
  edu.hk
• edu.hk return the IP address to hk
• hk returns the IP address to .
• . returns the IP address to client
• DONE !

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• Think about it, this kind of name-address
  management is very clever
• this means cuhk.edu.hk is responsible in naming
  computers and assigning IP addresses under its
  domain
• what is the domain again?
• cuhk.edu.hk
• if cuhk.edu.hk wants to remove www.cuhk.edu.hk,
  it can simply delete the entry from its
  directory, there is no need to inform the world

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• Likewise, if cuhk.edu.hk wants to add a new name
  to a computer, it can add a name to its directory
• e.g. www2.cuhk.edu.hk 137.189.23.43
• this computer can be reached immediately by
  clients all over the world
• the nice thing is that CUHK does not have to make
  a worldwide announcement about the IP address of
  this new web server

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• Now, about the chinese domain name, can you
  tell what is the main problem in popularizing the
  new Chinese domain name ??