The official website for The Settlers of Catan

Question about Catan - Cities & Knights for PC/MAC Can Catan – Cities & Knights also be played online?

Answer

The program Catan – Cities & Knights is only suitable for play against computer opponents. It doesn’t possess its own online mode.
However, the Catan Online World is part of the installation package. This program allows you to play the Catan board games and various other board games by Klaus Teuber online. You must register to use the program; to play the Settlers of Catan base game, all you need is a no-cost Free Account.
Further information about the Catan Online World can be found here.