How to setup two individual network in a router?

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http://serverfault.com – I am tryign to setup 2 networks for 2 rooms in my company. The first network is for upstairs and the second network is for downstairs. Upstairs users won't be able to see data in downstairs users computer (vice versa). Every computer is in the same subnet with static ip. They all connect to a switch -> router -> modem. Router: Linksys WRT54G Switch: Intellinet (Doesn't support vlan) Can I setup 2 workgroups to accomplish what I need? Any thoughts? Thanks a lot. (HowTos)