A MAC code is issued by your broadband provider and is used to switch from one ADSL broadband provider to another.

MAC stands for Migration Authorisation Code. It's basically a code that is needed by your new broadband provider to swap your connection over with minimal interruption. If you migrate from one broadband provider to another and you use a MAC code, you should only lose service for a few hours.

Please note: You don't need a MAC code unless you have an existing ADSL broadband connection (ie broadband through a BT line).