Can't find dns server!

[justnow]

Hi, all. I 've installed 2.2r0, and chose appropriate dns server ip during
the installation.
After the reboot, however, it can't resolve the hostname I request.(eg.
http://debian.linux.org.tw)
It seems that there is no dns server, because I still can use ftp service of
the debian.linux.org.tw via ip address.
Could anyone tell me how to configurate dns ?
Appreciate your reply!!