the csu/dsu has nothing to do with nat or any layer three protocols. You really dont need to use nat, imo, unless you are hiding your private IPs. Using NAT in an ISP sense saves IPs for the ISP, among other techniques, so unless you want to study how an ISP issues you your IP within its own "AS's" if you will, then set up as many NATs as you want. Or if you are getting a cert, then know NAT and PAT.
Is it getting bright in here, or am I just cool like that