Adding AirPort Extreme for a separated network?

Hi,

I'm trying to get a network setup in a small office, but the details have me swimming in the deep end with various issues (DCHP, subnet masks, etc.).

Objective:

Configure two separate networks that don't see each other and both provide WiFi, but share the same internet connection.

Setup:

Phone Line>

BEC (5200?) DSL Modem/Wifi Router (w/ 4 Cat5 ports)>

AirPort Extreme (7.7.3)

Dream:

Connect to the printer from both networks. The printer is currently hard-wired into one of the BEC ports.

Many thanks for any help (or clarifying questions).

Z