## Usenet

* Collection of mailing lists, commonly used for posting files * Archived and hosted by a network of ISPs and usenet providers * Decentralized file-sharing since 1980

## IRC XDCC

* Chatroom full of bots that announce which files they have * When you see a file you want, send a command to the bot and it sends the file * Sometimes have quotas too

## Napster

* Was a popular program for trading MP3 music files * Used central control server, but files sent P2P (like FTP's FXP) * Shutdown by music industry lawyers

## xor (⊕) * eXclusive OR * Fundamental boolean logic operator: ⊕ * Similar to addition, but no carry bits

Binary addition: 

  0 + 0  =  0
  0 + 1  =  1
  1 + 0  =  1
  1 + 1  = 10
carry bit ⤴

xor: 

  0 ⊕ 0  =  0
  0 ⊕ 1  =  1
  1 ⊕ 0  =  1
  1 ⊕ 1  =  0

## Redundant backups

* Say you have 2 hard-drives, A and B * Make a back-up by buying 2 more hard-drives for A and B * Now if you lose any one drive your files are still safe

## Parity backups

* You can save money by buying only one drive and storing `A⊕B` as the backup * Still safe against any one drive failure * Example: If drive B fails, recover with: A ⊕ (A⊕B)