Does this help?
http://static.highspeedbackbone.net/...e-e-Manual.pdf
If that's the same model as yours, then on the back you should have a row of network ports.
2 WAN ports (external), 1 Opt port (DMZ) and 3 LAN ports (trusted)
Seems to be that anything connected to the LAN ports can talk to each other, but access to it from the internet (WAN) can be restricted.
The Opt port would be for connecting something which you want to access internally and externally, without giving external access to your internal network, such as a web server or your NAS.