Pretty much everyone has downloaded from the iTunes store, but not too many of us know just how iTunes keeps track of computer authorization. Every device on the internet has at least two unique identifiers: a MAC address and an IP address.
MAC is an acronym for Media Access Control. Many believe that Mac, the abbreviation for Macintosh, should be written with capital letters – this is incorrect. Likewise, iPod – not iPOD or IPOD; iMac – not iMAC or IMAC; etc. Network interfaces have MAC addresses; Macintoshes can be called Macs.
Since your MAC address is completely unique, it’s the ideal way for iTunes to know that you’re authorized to play purchased content on any given machines. Trouble is, your ethernet port is part of the main logic board, which requires replacement in some repairs. With a new main logic board comes a new MAC address, which confuses iTunes and some other, generally high-end, software.
You’re allowed to authorize up to five computers at any one time to play your purchased content, but replacing your logic board changes the MAC address. If you didn’t de-authorize before repair, you’ve lost 20% of your available authorizations. I made this mistake a few years ago when I had to replace the logic board in a Mac Mini hooked up to my television, and when I sold my iBook. I also lost an authorization when my two-week-old PowerBook G4 flew off the roof of my car at highway speed. Thankfully, iTunes allows you to de-authorize all computers on your account once annually.
I only have one Machine these days, a 17-inch MacBook Pro, so this hasn’t been a problem of late for me. It’s a common question asked our technical support team, and a good fix to file in your troubleshooting arsenal.
The full details from Apple can be found here: http://support.apple.com/kb/HT1420