Business Data Communications textbook has two companion websites.  If
  you want to extend your knowledge in any of the areas covered in the book, we
  suggest you go through these two websites. For every chapter there are 
  sample quizzes and links. These readings are not mandatory for the class, but
  we believe you will find a lot of the material very insightful.
  http://cw.prenhall.com/bookbind/pubbooks/stallings/
  http://www1.shore.net/~ws/BDC3e.html