MAC 's (M.A.C) are used by "some" broadband xDSL (ADSL) ISP's as a line identification marker for individual customers, allowing for easier migration (moving) between different providers. These are normally issued by your ISP and then passed onto your future provider, speeding up the whole process.

Related Details:

http://www.ispreview.co.uk/new/complain/common_isp_complaints_migration.php#1