How to set up VLAN network

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http://serverfault.com – I'm changing my network from everything on one subnet to using VLans. My problem is that we already a lot of devices on this network already. From theory, I read that each Vlan has to be a different subnet and then I need to configure virtual interfaces on my Cisco router to cater for inter vlan routing. 1) How can I segment this network with minimum down time on the devices already on the network? 2) Can I just create Vlans and leave all these Vlans in the same layer 3 network so that they can go out of the network (I am not not concerned about Vlan routing) or I have to create subnets wh (HowTos)