Protect unauthenticated users to access network in DHCP mode

Hello All,

We are using DHCP to distribute IP address in our netwok but my problem is when anyone comes with laptop and plug into network port they get internet/intranet access without any autehtication.

Now we need to protect it the access of internet/intranet so is there any opensource software/solution for it.

Thanks and Regards,

Asger