system.net.webexception the operation has timed out
After doing google search I came to know that we can't use more than one connection for getting response from pages of same server.
So closing the connection using "Close()" after getting the response has resolved this issue.
And also, I came to know that just for finding whether a page exists, we need not get the all the response string. Using HttpWebRequest.method="HEAD" will get only the header part. This will improve the execution speed.