network user isolation

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http://serverfault.com – My question is for a network with a Linux iptables router gateway. How can it be possible to prevent inter-network traffic of those users. Think this case as a public network, IPs are distributed through linux gw and users are authenticated thru the gateway. We want to protect public users from public users. Network is not wireless and I can not use Wireless AP user isolation. Actually I have a simple method. Subnet the network into /30 mask. Give minimum IP of each subnet to the gateay and ditribute those /30 IPs from the subnet. But this is pretty costly for such an aim. I want to ask for (HowTos)