Here are some FREE external DNS addresses that you can use for testing.

Free Public DNS Server

Service provider: ScrubIt
Public dns server address:

67.138.54.100
207.225.209.66
Service provider:dnsadvantage
Dnsadvantage free dns server list:

156.154.70.1
156.154.71.1
Service provider:OpenDNS
OpenDNS free dns server list:

208.67.222.222
208.67.220.220
Service provider: vnsc-pri.sys.gtei.net
Public Name server IP address:

4.2.2.1
4.2.2.2
4.2.2.3
4.2.2.4
4.2.2.5
4.2.2.6