The /etc/nsswitch.conf file defines
the order in which to contact different name services. For Internet
use, it is important that dns shows up in the "hosts" line:
This instructs your computer to look up hostnames and IP
addresses first in the /etc/hosts file,
and to contact the DNS server if a given host does not occur in the
local hosts file. Other possible name
services to contact are LDAP, NIS and NIS+.
More in man nsswitch.conf.