How do you keep users from stealing other user's ip??

[Mark Jayson Alvarez]

Good day,
  
  
  We are trying to reorganize our local area network and I need some tips on how you are managing your own lan...
  
  We have a vanilla pc router with interface facing our private lan and interface facing the Internet.
  
 One problem which we are experiencing right now is that any user from private lan can use any ip address he wants. If he boots his computer with a stolen ip address, the poor owner of that machine(not active at the moment) will give automatically up his ip address to this user. The same scenario for public ip addresses. Basically, we need to track down the users through their ip address.. But this is trivial as of now since anyone can use any ip he wants. Even if there is a solution out there to tie up his mac address to his ip address..(sort of checking the mac first before giving him an ip, possibly through dhcp..) still, users can just download applications which will enable him to change his mac address....
  
 Now, where thinking about authenticating users before he is allowed to use a particular network service(internet proxy, mail etc.) because I guess it is a clever way of keeping the bad users from doing something bad within your network when after all, the reason why he is plugging his lancard to the network is to use a particular service. However, it still doesn't keep them from playing around and steal other ip addresses or mac addresses and thus denying network access to those legitimate owners. I'm thinking about tying dhcp with authentication, and freeradius comes to mind.. I just need some more tips from you. User's workstations are mixed Windows and *nixes. Some have laptops with wireless interfaces.
  
  Any idea how to handle this situations??
  Thanks...
  

		
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