MAC is short for Migration Authorisation Code, and was a code (17-19 character unique identifier) requested by a DSL broadband subscriber in the United Kingdom -- from their broadband provider -- when they wanted to switch providers. The MAC code system was originally a voluntary scheme in the UK -- which was signed up to by the majority of large broadband providers, such as BT, AOL, Sky, TalkTalk, Orange and Plusnet -- but Ofcom made it compulsory on the 14th of February 2007. Ofcom (Office of Communications) is a UK government approved regulatory body that has regulated the UK telecommunications market since the 29th of December 2003 (replaced Office of Telecommunications (Oftel)). The MAC code system was made obsolete on the 20th of June 2015, and broadband subscribers simple have to make a request (to switch) to their new ISP (who will handle the migration).
When the MAC system was 'live', to implement a DSL broadband connection on a new landline, it took up to two weeks to be activated. Likewise, if a subscriber wanted to cancel broadband on a landline (dsl tag is removed) it would take a similar amount of time of activate a new provider upon that line. The role of the MAC system was to seamlessly switch providers with no loss of service. Once the MAC code was requested from an Internet Service Provider, it would typically take up to five days to arrive by post (usually 2-3 days) -- the code was valid for thirty days. Once the subscriber gave the MAC code to their new Internet Service Provider, it would then, on average, take another five working days for the service to change over (3 days at best, 9 days at worst. While the subscriber waited for their new Internet Service Provider to take over the DSL connection, the old Internet Service Provider continued to supply a DSL service -- many ISPs rent their DSL service from BT's broadband infrastructure.
The Migration Authorisation Code was a 17 to 19 character unique identifier, that was a mix of letters, a slash and numbers, and typically began with four letters, followed by numbers (split with a slash). An example of a Migration Authorisation Code would be: FTIP92352342/DC23E. As stated, the MAC code, when issued, was valid for 30 days, and after that date a subscriber would have needed to request a new one. If a subscriber requested a MAC code, but changed their mind and didn't want to use it, then the customer was not required to contact their ISP, the code will become obsolete after 30 days and their broadband service would remain with their current provider. ISPs sometimes did refuse to provide a Migration Authorisation Code, but this was usually due to an existing issue with the subscribers account (it was already in the process of being canceled or the identity of the subscriber could not be verified).