Typically you can configure the router either as a NAT or as a bridge. What I'm looking for is some sort of hybrid mode that would allow some machines to pass through a NAT and use the router's DHCP and others to use directly my ISP's DHCP and IP address.
I'm fairly certain this can be done through routing rules and iptable rules I'm just not certain of the way to do it. The reason why I would like this ability is I currently have a VOIP adaptor plugged into a switch connected to my cable modem, and my router is plugged into the switch. I found the switch is no-longer working up to specifications and is slowing down my internet access. Rather than replacing it, I would like to find a way to plug in the ATA into the router without going through the NAT. (That NAT seems to cause intermittent problems with CID on outgoing calls.) I would also like to relocate the ATA to a different room and connect it though my homeplug network. I'm sure others would find this type of feature useful as well. Especially those with multiple static IP addresses allocated by their ISP.