Find IP address of a site using PING:

You can find the IP addr of a website by using PING command in Microsoft windows.

1.Press Win + R.
2.Type cmd   in the command space.

This will open the command prompt .

3.Type ping and your website whose IP has to be found.
  EX- c:\documents and settings\user>ping  www.yahoo.com  
and you will see the IP in brackets.

So the IP address of http://www.yahoo.com/ is 209.131.36.158 , it also shows that connection was successfully created as the no of packets sent and received is same without any loss.

 Basically Ping is a network tool used to test whether a particular host is reachable across an IP network.It works by sending   “echo request” packets to the target host and listening for  “echo response” replies. . Ping measures the round trip time and records any packet loss, and prints when finished a statistical summary of the echo response packets received, the minimum, mean, max and in some versions the standard deviation of the round trip time.

Some Other Links:
What is a Proxy Server?
What is an IP address?