websites not opening - 2

Ubuntu 13.04
Unable to open some websites one of them is https://twitter.com/

Google Chrome : Version 27.0.1453.110
The error is : Error 137 (net::ERR_NAME_RESOLUTION_FAILED): Unknown error.

Firefox version 21.0
Error is : Firefox can't find the server at twitter.com.

$ cat /etc/issue
Ubuntu 13.04 \n \l