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

Sten Daniel Sørsdal lists at
Fri Mar 24 09:39:33 UTC 2006

Mark Jayson Alvarez wrote:
> 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 still other ip addresses or mac addresses and thus denying network access to those legitimate owners.
>  Any idea how to handle this situations??
>  Thanks...

If it's a service provider scenario i would employ vlans. One vlan to 
each customer. Providing network or Internet service costs more than 
your typical small company network. Each customer should get his/her own 
dedicated "line" so to speak.

I would most likely employ /30 networks (or larger) to each customer as 
this would be the most solid way to do it. This goes for public IP 
addresses as well. You could bridge the vlans but this will give you 
grief and if not done right will leave you back at square one.

Some would say PPPoE, which is a fine solution. It comes with it's own 
set of challenges. Many idiotic hobby "admins" out there block icmp all 
together. Some even drop fragments. But

Managed vlan switches are becoming quite affordable these days. Not only 
would they help you track down a "sinner" within minutes (instead of 
hours, if not days). They often come with more than adequate snmp 
support so you can do real monitoring (even the low end ones).

Sten Daniel Sørsdal

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