Correct. Only Business Class subscribers can get a statically assigned WAN / public IP addy.
And no. That can't be specified.
Are you talking about a static external IP address? If so, skip this 🙂
If you are referring to internal IP addresses handed out by the xfinity router (DHCP), then it is impossible to simply mark an IP as reserved.
The only work around I have found is to setup port forwarding with an unused port to that device/IP. Then xfininty saves the reservation.