said by mario02423:
Call the Business Class support number and request that your SMC be put into bridge mode. This will pass all internet traffic through to the R6200, and you can manage the R6200 like you would a residential router. Things will be much easier for you this way.
If the OP does that, (s)he better be prepared for Comcast CSRs to randomly do a reset on it and take it out of bridge mode...and then having to call back multiple times (wasting many hours) to get it put back into bridge mode. This is the voice of experience talking.
If the OP has cascaded NAT problems using the SMCD3G-CCR with the R6200 in the SMC's pseudo DMZ, the best bet would be to purchase a Motorola SB612x and use that as the modem for the business class connection.
The SB612x is officially supported as a business class modem, and if it is a new modem (not a used modem from Craigslist or eBay), there should be no problem just plugging it in and activating it through the walled garden activation web site.
Two added advantages of using an SB612x on a business class connection are that (if your CMTS supports IPv6) you can get Comcast's dual stack IPv4/IPv6 (not currently possible with the SMC gateway in router mode), and you will have up to 5 public IP addresses available (which is handy for setting up VoIP, guest WiFi, etc for Internet access (but otherwise totally isolated from your LAN).--
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