In Chapter 34, you used a "homemade" Web based on the Winsock APIs. In this chapter, you'll learn how to use and extend Microsoft Internet Information Server (IIS) 4.0, which is bundled with Microsoft Windows NT Server 4.0. IIS is actually three separate servers—one for HTTP (for the World Wide Web), one for FTP, and one for gopher. This chapter tells you how to write HTTP server extensions using the Microsoft IIS application programming interface (ISAPI) that is part of Microsoft ActiveX technology. You'll examine two kinds of extensions: an ISAPI server extension and an ISAPI filter, both of which are DLLs. An ISAPI server extension can perform Internet business transactions such as order entry. An ISAPI filter intercepts data traveling to and from the server and thus can perform specialized logging and other tasks.