iBook Not Finding DNS

Kathy's iBook quit finding things on the internet. For some reason it did not know how to locate DNS (our nearest internet phone book.) The wireless router is supposed to tell the iBook where DNS is located.

She thought she had lost internet access, but this would work:
http://216.109.118.79 (Yahoo's IP address)
when this would not:
http://www.yahoo.com

I manually set the i.p. addresses for our two nearest DNS servers:
4.21.254.24 4.21.254.2 (dns1.netcentrix.net)
4.21.254.25 4.21.254.3 (dns2.netcentrix.net)
in the Network system preference.