Get external IP address

You behind a router and want to know the external IP.
Use:

nslookup myip.opendns.com. resolver1.opendns.com

from the commando prompt or via a script.

It will return something like:

Server:  resolver1.opendns.com
Address:  208.67.222.222

Non-authoritative answer:
Name:    myip.opendns.com
Address:  12.34.56.78

Where 12.34.56.78 will be your External IP address.

Thanks to http://superuser.com/questions/165986/windows-command-that-returns-external-ip