Open a console and try running these:
$ ping -c3
www.yahoo.com
PING
www.yahoo.akadns.net (216.109.118.65) 56(84) bytes of data.
64 bytes from p2.
www.dcn.yahoo.com (216.109.118.65): icmp_seq=1 ttl=51 time=34.3 ms
64 bytes from p2.
www.dcn.yahoo.com (216.109.118.65): icmp_seq=2 ttl=51 time=34.9 ms
64 bytes from p2.
www.dcn.yahoo.com (216.109.118.65): icmp_seq=3 ttl=51 time=38.7 ms
$ ping -c3 216.109.118.65
PING 216.109.118.65 (216.109.118.65) 56(84) bytes of data.
64 bytes from 216.109.118.65: icmp_seq=1 ttl=51 time=37.3 ms
64 bytes from 216.109.118.65: icmp_seq=2 ttl=51 time=33.3 ms
64 bytes from 216.109.118.65: icmp_seq=3 ttl=51 time=38.0 ms
If you can ping the numeric ip but not
www.yahoo.com you don't have your domain name server setup correctly. That's configured in /etc/resolv.conf and should look like this:
nameserver <ip address of your isp's name server>
nameserver <ip of your isp's second name server>
search <ispname.com or .net>
The ip addresses in the above are numeric and should be available from your isp.