Tomato/DD-WRT router to act as switch & only NAT some port
- by fseto
BACKGROUND:  I have a device that must use a real IP address.  Currently, my ISP uses DHCP and I can have up to 4 real IP address assigned.  However, the cable modem only have 1 ethernet port and it's connected to my router (running Tomato, but can run DD-wrt or other Openwrt if required).  Question stems from how I can connect the additional device, requiring a real IP?
EASY SOLUTION:  would be to get a switch and connect to the CM, Router, and Device.  But alas, I want to avoid this route, since:
my wiring cabinet in my home is drawing lots of power and heat already
Device will be unprotected by any firewall
unable to monitor the traffic to/from device.
Besides, what would be the FUN in that? =)
IDEA:  So what I want to do is to configure the router, so that one of the switchport is removed from the normal br0 bridge.  Instead, I want to make it behave like a switch on the WAN port.
What's the best way of doing this?  Should I create another bridge on the WAN & the device port?  Can a single port belongs to two bridges? or would I need to create a subinterface first?  Would I need a DHCP-relay?  Am I expecting too much from my poor cheapie router?
   +------+
   |  CM  |
   +--++--+
      ||
+----WAN---------------+
|   /    \      Router |
| BR1?   BR0           |
|  |       \           |
|  |       {NAT}       |
|  |     / |  | \      |
+-P0----P1-P2-P3-Wifi--+
   |
+------+
|Device|  
+------+