how to access a modem behind a linksys WRT router?

I’ve got a versalink 327W dsl router/modem combo that I have plugged into a linksys router’s “internet” ethernet port. I’m not sure what kind of internal ip address is assigned to the device in that port, and I would like to be able to access the modem’s web interface through the wrt router with dd-wrt firmware installed if possible.