|
Network Programming  This website demonstrates using wikis as teaching and learning tool. The course instructor is happy to share the teaching materials here with those who find it readable. |
Lecture /
Introduction to NetworkingA Network Programming Lecture by Steven Choy Lecture Overview: Networking Stacks and Layers - Internet Protocol - IP Addressing -- IP Addressing - for Private Networks - IP Subnets - TCP (Transmission Control Protocol) - UDP (User Datagram Protocol) - Domain Name System - Firewalls and Proxy Servers - Internet Protocol version 6 - Overview of basic concepts for network programming
Networking Stacks and Layers
Internet Protocol
Windows has a program that lets you trace a packet movement between routers. Type tracert <somehost> in the command prompt. Replace <somehost> with some hosts you want to test.
If you are using Windows, type ipconfig or ipconfig /all in the Command Prompt. It will then list the host name, IP address, subnet mask, gateway, and even the MAC address of your network card.
Public versus private IP addresses: Besides the reserved IP addresses (0.0.0.0/8 and 127.0.0.0/8) mentioned above, there are other addresses not used on the public Internet. These private subnets consist of private IP addresses and are usually behind a firewall or router that performs NAT (network address translation). NAT is needed because private IP addresses are nonroutable on the public Internet, so they must be translated into public IP addresses before they touch the Internet. Private IPs are never routed because no one really owns them. And since anyone can use them, there's no right place to point a private IP address to on the public Internet. Private IP addresses are used in most LAN and WAN environments, unless you're lucky enough to own a Class A or at least a Class B block of addresses, in which case you might have enough IPs to assign internal and external IP addresses. (Extract from IP subnetting made easy)
If you use a router/proxy to connect to the Internet, your IP address as seen by a host in the Internet is a public IP address. To know the public address you are using, try the following websites.
IP addressing
1. What is the class of the network for each of the following IP addresses?
(a) 12.12.12.12
(b) 125.44.25.223
(c) 220.32.34.125
2. Why there is no network that owns an IP address such as 127.x.x.x?
3. Can you send data to 234.5.6.7? If so, what will happen?
IP Addresses for Private Networks(Extracted from IP Addressing - Linux Knowledge Base and Tutorial
In some cases, there is no need to have unique IP addresses, since the network will never be connected to the rest of the world. For example, in a factory where the machines communicate with each other via TCP/IP. There is no reason for these machines to be accessible from the Internet. Therefore, there is no need for them to have an official IP address.
You could just randomly assign IP addresses to these machines and hope that your router is configured correctly not to route the packets from these machines. One slip and you have the potential for not only messing up your own network, but someone else's as well.
The solution was provided in RFC-1918. Here, three sets of IP address were defined for use in "private" networks. These won't be routed and there is no need to coordinate their use with any of the registrations agencies. The IP addresses are:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
As you can see that there is just a single class A address, but 16 class B and 255 class C networks. Therefore, no matter what size your network is, you can find a private network for your needs.
IP subnets
Using the subnet technique, network administrators can divide a network into multiple subnetworks and connect subnetworks with routers.
IP Datagrams
TCP (Transmission Control Protocol)
UDP (User Datagram Protocol)
IP family of services
Telnet is a service that allows users to open a remote terminal session to a specific machine. This allows Unix users, for example, to access their account from terminal servers, or desktop machines. Since Unix servers are intended to support multiple users, a telnet session is often used as only one person can access the machine from the local terminal (using a keyboard and monitor). Telnet allows many users to connect over the network, and to access their accounts as if they were doing so locally.
The ability to transfer files is extremely important: even before the World Wide Web, people distributed images, documents, and software, using the File Transfer Protocol (FTP). FTP allows a user to login (using a special username and password), or to attempt an anonymous login (by using the username of anonymous). FTP servers will often grant different access permissions depending on the user. For example, an anonymous account might be unable to write a file to the server, but may be able to read all files. FTP uses two TCP ports for communication – port 21 is used to control sessions and port 20 is used for the actual transfer of file contents.
Email has become a vital part of most people’s lives. With the exception of web-based email, or specialized accounts, the majority of people access their email using the Post Office Protocol, which uses TCP port 110. Messages are stored on a server, retrieved by an email client, and then deleted from the server. This allows offline reading of mail, without being connected to the Internet.
While many browsers and email clients support only POP3, some also support the Internet Message Access Protocol (IMAP). This protocol is less popular, as it requires a continual connection to the mail server, and thus increases bandwidth consumption and disk usage since messages are not stored on the user’s system. IMAP allows users to create folders on the mail server, and also allows online searching of mail.
The Simple Mail Transfer Protocol allows messages to be delivered over the Internet. You might find the separation between retrieving mail, and sending mail, a little strange. However, it actually simplifies the process considerably, and allows different mail retrieval protocols to be used, or custom mail accounts.
HTTP is one of the most popular protocols in use on the Internet today, and made the world-wide-web possible. HTTP is an extremely important protocol, and Java includes good HTTP support.
The WHOIS protocol allows users to look up information about a domain name. You can find some surprisingly useful information by doing this, such as the address of a company, who registered the domain name, and contact details for the registration.
(Extracted from Chapter One - Networking Theory, Java Network Programming & Distributed Computing, by David Reilly and Michael Reilly (ISBN: 0201710374))
Domain Name System
Firewalls and proxy servers
 (Source: Java Network Programming & Distributed Computing, by David Reilly and Michael Reilly)
(Reference: Firewall Q&A )
Internet Protocol version 6
Improvement by IPv6
IPv6 Information Page (www.ipv6.org) - http://www.ipv6.org/
RFC Sourcebook for IPV6- http://www.networksorcery.com/enp/protocol/ipv6.htm
Overview of basic concepts for network programming
Sample dialogue of application-level protocol: POPPOP Protocol: The following shows you a sample dialogue between a POP client and server. You can use "telnet pop.netvigator.com 110", for example, to test them. For testing purpose, you need to find the POP server for your own email account. (For examples, popmail.ouhk.edu.hk and pop.netvigator.com are two POP3 servers that you may have accounts.) Server: +OK InterMail POP3 server ready. Client: USER sochoy Server: +ok please send PASS command Client: PASS ********* Server: +ok sochoy is welcome here Client: STAT Server: +ok 213 8404352 Client: LIST Server: +ok 213 message 1 2438 2 2963 ... 213 937 Client: DELE 1 Server: +ok Client: QUIT Server: +ok sochoy InterMail POP3 server signing off. Sample dialogue of application-level protocol: SMTP
Simple Mail Transfer Protocol (SMTP) is the protocol for sending email from one server to another. It is also used to send email from a mail client to a mail server. SMTP uses TCP port 25. One good way to visualize SMTP is to do some real interactions with an SMTP server. The following activity will guide you to do this experiment.
This activity is about how to use a command prompt on Windows XP to connect to an SMTP server and communicate with it about sending an email message. Please follow the steps below carefully.
To do the following activity, you need to know the hostname of an SMTP server that you can access from your computer. You can find the information from your mail client (also know Mail User Agent) such as Microsoft Outlook Express. The information can be found by checking the value in "Outgoing mail (SMTP)" within the account setting.
In the following description, smtp.ouhk.edu.hk will be used as the SMTP server for demonstration purpose.
There are three steps to SMTP mail transactions. The first step is the sending of the MAIL FROM and RCPT TO messages by the sender, which give the sender's identification and the receiver's information. The second step is sending the DATA message, which gives the mail data. The last step is confirming the transaction by sending the end of mail data indicator.
Step 1. Open a command prompt on Windows XP and type:
telnet smtp.ouhk.edu.hk 25 You should receive a response message from the server. For example, I received the following:
220 ouhk.edu.hk ESMTP Sendmail 8.11.1/8.11.1;
Fri, 30 Sep 2005 17:43:23 +0800 (HKT)
Step 2. Type MAIL FROM: followed by your email address. For example, I entered the following:
MAIL FROM: andy@ouhk.edu.hk Step 3. You should receive a response message from the server. I received the following message:
250 2.1.0 andy@ouhk.edu.hk... Sender ok Step 4. Type RCPT TO: followed by an email address that you can access for later verification purposes. For example, I entered the following:
RCPT TO: betty@stevenchoy.com Step 5. You should receive a response message from the server. I received the following message:
250 2.1.5 betty@stevenchoy.com... Recipient ok Step 6. Type DATA as shown in the following.
DATA Step 7. You should then receive a response message from the server similar to the following:
354 Enter mail, end with "." on a line by itself Step 8. Type the message you want to send for testing. Press the "return" key when you finish a line. Finally, type "." to tell the server that is the end of the message. (A new line containing only "." means the end of the message body.) The following is what I entered in my testing:
This is the first line of testing message.
This is the second line of testing message.
.
Step 9. The server should respond with some appropriate message like the following.
250 2.0.0 j8U9hrH01679 Message accepted for delivery Step 10. Type QUIT to close the connection with the SMTP server.
Finally, use your mail client program to log in to the receipt email account and fetch the testing email. This is to verify that what you have done from the above steps has been successful.
Thanks for ReadingIf you would rather like to have this lecture note in printed format, please click the print action link in the top right corner. If you find any problem in this lecture note, please feel free to tell Steven via steven@findaway.hk. |
