Why do you Need IDS?

Thank ged.At last you have answer my question.Cheer...

Can IDS detect DDoS attacks? if yes, what's the working methodology?

Depends on which specified IDS she is. Normal marketable nowaday IDS could sense/detect about more than 55 basic attack signatures, which includes most basic attack techniques. By eliminating the changes of these basic attack, IDS ensures no other morhp of the attack could occur.
IDS is different from Firewall in one point,i think.
Firewall are built to control, filter the access of the network, protect network from illegal access while IDSs are more specify for anti-hacking task.

I wrote a nice long tutorial on IDS's here it is,
http://www.antionline.com/showthread...hreadid=242664