Couldn't Connect to External Sites

My WordPress blog is unable to connect to external sites.
The below quote text can be seen when i am trying to install a new plugin directly from wordpress panel.

An unexpected error occurred. Something may be wrong with www.ads-software.com or this server’s configuration. If you continue to have problems, please try the support forums.

This one shows under the WordPress News of Dashboard

RSS Error: WP HTTP Error: Couldn’t resolve host ‘www.ads-software.com’

I tried reinstalling my server but that doesn’t work.And i even i tried that plugins thingy by disabling all,I was disappointed with too.
I am able to ping other sites from my SSH and I checked resolv.conf – it looks fine too.
Please help me to solve this problem.

Thank You