This book assumes you are comfortable with the Java language and programming environment, in addition to object-oriented programming in general. This book does not attempt to be a basic language tutorial. You should be thoroughly familiar with the syntax of Java. You should have written simple applications and applets. You should also be comfortable with basic AWT and Swing programming. When you encounter a topic that requires a deeper understanding for network programming than is customaryfor instance, threads and streamsI'll cover that topic as well, at least briefly .
You should also be an accomplished user of the Internet. I will assume you know how to FTP files and visit web sites. You should know what a URL is and how you locate one. You should know how to write simple HTML and be able to publish a home page that includes Java applets, although you do not need to be a super web designer.
However, this book doesn't assume that you have prior experience with network programming. You should find it a complete introduction to networking concepts and network application development. I don't assume that you have a few thousand networking acronyms (TCP, UDP, SMTP, etc.) at the tip of your tongue. You'll learn what you need to know about these here. It's certainly possible that you could use this book as a general introduction to network programming with a socket-like interface, and then go on to learn WSA (the Windows Socket Architecture) and figure out how to write network applications in C++. But it's not clear why you would want to: as I said earlier, Java lets you write very sophisticated applications with ease.
