When conducting this investigation, I found a really intersting and at the same time very dangerous thing. That is, it is damn easy to catch the data on the network which is sent to the other people!!
I use "FreeBSD". This system hires a useful tool titled "ports collection". By using this tool, we can install various softwares to our FreeBSD rather easily. There is a software titled "sniffit" in FreeBSD ports collection. This software is made in order to "peep" the packets' header. Actually it is VERY EASY to get other people's password when they use FTP or telnet.
You can realize how dangerous it is to use FTP or telnet, just by using this software. I recommend you to try this interesting activity :-)
On the other hand, we can use this technology to a more constructive way. You know, in Windows world, we cannot get to know exactly how our applications works, since the source code is not distributed. However, if we use this packet stealing technique, it might be possible to get to know exactly what is occuring when we use Windows application. However, many people will deny this fruitful result by saying "it is against the manner to peep ..." :P