Hi,
things have changed from when you have to add nameserver in file /etc/resolv.conf
In case of a static configuration in /etc/network/interfaces is possible to add nameservers right here, with the following syntax
English
dns-nameservers 8.8.8.8 213.205.32.70
As you can see is possible to define multiple nameservers. I will use Google public DNS for resolution because it run faster than my internet provider DNS server.
If you want to check speed of the nameserver:
dig @8.8.8.8 www.google.com