A Web server is a program that using the client/server model and the World Wide Web's Hypertext Transfer Protocol (HTTP). It serves the files that generates Web pages to Web users (whose computers contain HTTP clients that forward their requests). Every computer on the Internet that host a Web site must have a Web server program. Two leading Web servers are Apache , the most widely-installed Web server and Microsoft's Internet Information Server (IIS). Other Web servers include Novell's Web Server for users of its NetWare operating system and IBM's family of Lotus Domino servers, primarily for IBM's OS/390 and AS/400 customers.