This Knowledge Base article will show you a few ways to determine what your external IP is as well as showing you what type of IP addresses cannot be claimed as external networks.
Checking your current External IP
There are a few ways we suggest when you want to check what your current External IP is so that you can add it as a Network.
1. The first is located at the top of your OpenDNS Dashboard as shown in the image below and is always present on your Dashboard.
2. Another option is to visit the URL below and it will open a blank web page which contains your External IP in the top left.
3. The third option would be to use the third party website that is listed below. Visiting this site will provide you with your External IP.
There are a few groups of IP address ranges that cannot be registered as they are Private non external IPs that are outlined in the RFC-1918 Article. The address ranges are listed below so that you can make sure you aren't trying to implement an address that can't be used.
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255