Question : Users Ilegal access to network resources

Hi, Tanks in advance for any help.
On my company we have a network on a NT domain Environment (we don't use Active Directory). The workstations (XP Pro) receive the IP Addresses from the DHCP server (dynamicaly). The network address is 192.168.150.0 and the Machines receive IP's from the scope 192.168.150.60 to 192.168.150.254, the other IP Addresses are reserverd  for Servers,Network Printers,Routers,Switches,etc.. The internet access is controled by a "proxy"  that enables or disables access by IP Address. I've detected recentely that apparently some users (againts the company rules), fake their IP Address, MAC Address and PCName,  in order to gain Internet access. On the DHCP Server I see the same PCName with several entries ( several IPs with same PC name and diferent MACAddress). I suspect that the users try to do this using personal computers (laptops) that don't belong to the Domain. I appreciatte any help that you can get me, in order to prevent or to track this ilegal procedures.

Answer : Users Ilegal access to network resources

Use NTLM proxy authentication. In order to get access to the Internet, browser will need to send username and password from approved computers with a correct SID.