hi,
i am a student and i would like to learn how to write
a network monitoring program(passively).plz suggest me
the algorithm with code,so that i could understand it easily.
bye

There are many GPL programs that do this. For example, look for "snort" to see how it is done.

This could be one of the Basic starts of learning how to write Network Monitoring program..

http://ciac.llnl.gov/ciac/ToolsUnixNetMon.html

http://www.slac.stanford.edu/xorg/nmtf/nmtf-tools.html

Suggestion.
* Angel is a simple yet useful tool to monitor the services on your network. Technically speaking, it's a Perl program that runs every 'n' minutes (usually fired from your cron) and calls different perl subprograms (referred as "plugins" ) to do the actual testing. It will then generate an HTML table containing the status of your network.

The CMU distribution for SNMP is a great place to start if you really want to master network management.... you need to master:

• SNMP
• UNIX Syslog
• The C Programming Language
• PERL, PHP, PYTHON or other SCRIPTING language.
• TCP/IP client-server......
• and a whole lot more !!!