Alert when workstation plugged into network

i am trying to find a way to alert me when a new workstation is plugged into company network jack to obtain an IP address
(via DHCP)  Right now, guest and employees bring in their own home laptops and connect to network to get an IP and surf
the Internet.  I don't know what in their laptops (virurs, spyware....) I have been checking on Cisco NAC but they are too expensive.
Do you know any alternative way to alert me when new one is plugged in.  Right now I just have to check the DCHP for address
leased in order to find out.
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802.1x provides port security without the expense of NAC.
bluechip99Author Commented:
can you please be more specific?
802.1x is a configuration on the switch that authenticates the computer (usually against a certificate). If the computer fails authentication, the port prevents access  

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DonNetwork AdministratorCommented:
This may be an option for you

How to Filter MAC Address with Windows Server 2003/2008 DHCP Server Callout DLL
This article gives a great overview of 802.1x
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Network Management

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