Authentication Bypass

Hello Untangle friends. I have setup Untangle and its working well. I have one issue. I notice the captive portal tracking clients by IP not mac address. So does it mean I can authenticate on device A and if I know the IP address of Device A, since my timeout has not yet been reached I can now disconnect device A from the network, then use the same IP address for device B to bypass authentication?

Anyone help will be appreciated.