hi, thanks everyone for the answers
@Willoughby I don't think the mac address is sent/received in an http request, isn't ?
@dave your suggestion is in the right direction but how can you automatically tells the browser to send this token?
@niphlod authentication is not enough, an user can still use his/her credentials to log in in a different tablet, ip address would work but it is too intrusive for an already present intranet.
Following @dave suggestion, I thought about a cookie that never expire, the first time the tablet start the 'a setup app' stores the cookie on both the client browser and in the server, however I'm not convinced this solution would work either; the 'setup app' is not foresee at all. I've to verify if there're ways to pre-install the cookie in the tablet?