Ok, you have an unknown model Mac running System 8.6 (or is it 8.6.1?).
How did you ping your Mac from the unknown (manufacturer and model number) router? Or, did you just access the router's internal server (web site) and look at the 'Attached Devices' (or similar) web page?
'But i cannot ping it from any other m/c (Microsoft Win98/XP Platform.) on my network.', ok ...
'So i am not able to connect it to my FTP Server.' ... 'Can someone help me about this????', well - is the 'my FTP Server' on the unknown model Mac running System 8.6.x?, or is it on one of the Windoze based PCs?, or is it your ISP's FTP server (of which your account allows you access to)?
If the latter, then who is your ISP?
Also, regardless of the ping'ing and the attempted FTP'ing, can the (unknown model) Mac connect to the internet (view web pages, etc.)?
Please note: I am not assuming that the System 8.6.x based (unknown model) Mac is what was used to post 'Unable to ping MAC' with.