1. Department of Information Engineering 1 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

2. Department of Information Engineering 2 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

3. Department of Information Engineering 3 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.com:2000/ which means please send this request to www.xyz.com with port number 2000

4. Department of Information Engineering 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. Department of Information Engineering 5 Use of port number Proxy server –Internet access is slow, can we speed it up? client routerWWW server Campus LAN fast (100Mbps) inexpensive Oversea Link slow (3Mbps) expensive Bottleneck

6. Department of Information Engineering 6 What is a proxy –a agent (software) that helps you to get the data client routerWWW server Bottleneck proxy Big disk cache

7. Department of Information Engineering 7 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. Department of Information Engineering 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 -> select preference -> advanced –specified the name and port number of proxy proxy.cuhk.edu.hk at port number 8080

9. Department of Information Engineering 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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11. Department of Information Engineering 11 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. Department of Information Engineering 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

13. Department of Information Engineering 13 What is a firewall? –A security measure that separate the computers of the company from the outside world Firewall Internet Company LAN

14. Department of Information Engineering 14 Firewall is just a server that has the responsibility of protecting the company’s 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

15. Department of Information Engineering 15 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. Department of Information Engineering 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

17. Department of Information Engineering 17 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 ‘. ’

18. Department of Information Engineering 18 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. Department of Information Engineering 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. Department of Information Engineering 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’

21. Department of Information Engineering 21 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 !

22. Department of Information Engineering 22 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

23. Department of Information Engineering 23 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

24. Department of Information Engineering 24 Now, about the chinese domain name, can you tell what is the main problem in popularizing the new Chinese domain name ??