If you can ping with only the ip address but not with a domain name like yahoo.com, then your nameservers are not setup properly
i will show you how to fix this:
LINUX:
edit the file /etc/resolv.conf. if you are the command line type: vi /etc/resolv.php or if you are a beginner like me, type: nano /etc/resolv.conf
this will open the resolv.conf file and enter your dns server from your ISP:
search yourdomain.com nameserver 202.54.1.10 nameserver 202.54.1.1
Where,
WINDOWS:
If you have window, I will be using windows XP, ( also works for windows 2000 )
Find out DNS server IP under Windows XP/NT
Click on Start button > Run > and Type command cmd > Press enter key
At DOS prompt type the command:
C:\>ipconfig /all
It will display DNS server IP address, and other information related to Windows networking: