You definetly have a public IP though. How often it changes will depend on your provider. I assume since you have Charter you are in the US and are using Cable Internet.
Most likely your IP will not change that often. You can check it on
http://www.whatismyip.com/
You could then look at whatever you are using internally to connect. for example
http://localhost:89/index.view you would replace the localhost with the IP address you found above.
If you have more then one PC connected to your Cable Internet you will have to setup port forwarding on your router. You also may need to open the port you are using in Subsonic on your Windows Firewall (assuming you are on windows).
IF you need someone to test externally you can PM me your IP address and link and I will be happy to try to hit it from the Internet.
Edit: Looks like I was too slow. The link above is a good discussion on this topic.