hello,
My question is..,
i have a FTP server on FC4, what i have to do if i dont let people outside the Network to use FTP server even if they have the the username and password of FTP account?
(please keep in mind that im very much new with Linux environment )

outside of what network? you need to tell us what defines "outside" and what devices are involved in that. it's *probably* a case of you going to portforward.com and reading up about your adsl router.

Ill assume you mean you want to let people access your FTP network over the internets. Well what you need is a web address so your FTP has a DNS identifier that way computers can find it over the internets. The other day Lifehacker had a nice article regarding how to setup a FileZilla server and this included how to access over the internet.

In a nutshell you can get a free domain from DynDNS.org and then download the DynDNS updater and install it, that program updates the site with your IP so that your domain name directs to your server. Im sure i left something out but thats about it.

ok guys i think i nead to clarify my question,
see i have a FTP server on FC4(this machine is live as well), if got FTP account information its username and password then i can access from any where in the world because this is live server, what i want to do is that i dnt want anyone to access my FTP server over the network, to access it he/she must be physically present in office LAN. in short i want to limit its access to LAN only.