6100D external access

I have a 6100D with a public IP address.  I've successfully set port forwarding for four ports on one internal IP address (192.168.1.x).

I've tried to forward two other ports on a SECOND internal IP address (192.168.1.y).  I'm permitted to add the IP address and ports, but in a short time, the IP address is CHANGED by the router to the first IP address.  I can make a connection before the IP address changes, but it disconnects after the change.  I don't know why the second IP address is not retained- the port is retained, however.  Can someone help?