If you got error when trying to ping any web address e.g. www.google.com, but can ping its IP address, there could be something wrong with the resolv.conf.
Despite you could have set the dns-nameservers in /etc/network/interfaces, the resolv.conf may not be updated yet. And if you have some running process and could not do ifdown / ifup, you can do nano /etc/resolv.conf, comment the original nameserver 127.0.0.1 to nameserver 184.108.40.206 (or the DNS provided by your ISP).
Ubuntu should now resolve the web address correctly!