This should be set for you already, we either use Google DNS or OpenDNS nameservers which are:
OpenDNS: 208.67.222.222, 208.67.220.220
GoogleDNS: 8.8.8.8 and 8.8.4.4
If you are writing it manually you need to set your /etc/resolv.conf to
nameserver: 8.8.8.8
nameserver: 8.8.4.4
Please note, if you do not know the resolve IP's please contact support