|"...I imagine that millions of home networks must have the same I.P. address..."|
Absolutely not! Every public IP address is unique. That is how all information is sent via the internet. If there were duplicate IP addresses then there would be no reliable way for data to be sent via the internet.
Port 21 is for FTP access. So, FTP support may already be enabled on the Buffalo Station.
First, try accesing the buffalo station via FTP on your network by using the "internal" ip address of the buffalo station. ftp://internalip
If that works, then your next step would be to make it available externally. You would need to forward port 21 in your router to the buffalo station. Then you would be able to access it from a remote computer using the "external" ip address that is assigned to your router.