Check to see if you ip from ipconfig/ifconfig and from a website such as whatismyipaddress.com gives the same ip.
You could be under an ISP NAT.
I was always kind of hoping that this wouldn't be the case. I ran ipconfig and my public IP wasn't listed anywhere. I get the typical 192.168.XX.XX. This is all while bypassing my router and plugging directly into the modem.
If this is the case, and my ISP is providing NAT services, is there anything I can do? Also, is there a way that I can verify that this is absolutely the case, and not something else?
Update: Pretty sure the issue is with my modem. It has it's own NAT that I need to work around. I'm in the process of trying to find the default username / password that my ISP uses for this modem right now. I'll keep this post updated.