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I need to lockdown unused IP addresses so people cannot access the network just by plugging in. The computers have static ip's but if I were to plug my personal laptop into a cable I now have network access and it is pulling an IP from somewhere and that is what I need to prevent.

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Physical security of small networks is much easier than NAP. – Chris S Jul 20 '10 at 13:06
I suggest also investigating the arpwatch utility if you're using Linux. – MikeyB Jul 20 '10 at 15:08
Please tell us a little more about your set up. If your machines have static IP addresses then do you even have a DHCP server? If you do, do you need it? Could you just turn it off? – Justin Ohms Jul 16 '11 at 4:02

The strong-armed solution for something like this is 802.1X, where a user has to authenticate to gain access to a network port.

Implementing this is non-trivial.

How large is your userbase? If it's reasonably small (or if you have a good up-to-date inventory of user machines), you can configure your DHCP server to only hand out addresses to known machines.

What are you using for your DHCP server right now?

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If you have DHCP running on a Windows Server 2008 (or R2) you can use DHCP NAP to prevent "rogue users" from obtaining an IP. There are several other ways to secure a network using NAP, including IPSec or 802.1X; and it can be implemented over VPNs as well.

As Matt Simmons pointed out, it's not trivial. See the NAP document from this download page.

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A simple solution if you are using DHCP with IP reservations, would be to reserve all unused IP addresses to bogus MAC addresses.

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