Cisco Switching/Routing :: NAT Multiple External IPs On RV180?

how to properly route multiple external IPs to internal ips assigned to several servers.  Where I'm getting a bit lost is that I have two levels of routing - one from the outside into a comcast business class cable modem, which also creates its own internal network.  Within that internal network, I have my RV180 router, which creates another separate internal network in which the servers live (don't want the servers to have access to other computers within the Comcast Modem's internal network).  I want to map the external ips to servers within the RV180 second level network.
 
To diagram:
 
Comcast Cable Modem
/         
RV180     Computer(s)
/    
S1     S2
  
An internal IP is assigned to the RV180 router.  Suppose it is 192.168.1.100.
 
The cable modem itself supports NAT as well.  Do I use NAT twice (on both cable modem and RV180) to route the external IP to S1 and S2?
 
Or do I modify the routing table on the cable modem?  Then use One to One NAT on the RV180 to map the external IPs directly to the internals?