IP Passthrough means the CPE device terminates the DSL, authenticates with the network (Receives a WAN IP) and shares that IP address with a single device connected to the CPE equipment.
Bridged Mode means the DSL/Broadband modem/gateway device only terminates a DSL connection. In a Bridged Mode the modem/gateway device does no authentication, does no management, has no ability to perform any firewall protection, and does not allow for remote access into the modem/gateway device. The subsequent device connected to the DSL/Broadband CPE will have to perform all these tasks if needed.
I was too lazy to type all this so I cut and pasted from
HERE.