A MAC is generated by the actual telecommunication provider (most commonly BT), identifies the local loop (telephone line) to be switched, and authorises the provider to switch the customer to the new ISP.
MACs are valid for 30 days once issued, providing the customer with the opportunity of switching ISPs within that period.
It usually takes around three to ten working days for accounts to be switched over from one ISP to another.
This meant that if a broadband customer requested a MAC from their ISP then the provider must supply one free of charge.
As of 20 June 2015 the MAC code is no longer required to enable switching of ISPs.