Preface For more than 40 years, IBM® mainframes have supported an extraordinary portion of the world's computing work, providing centralized corporate databases and mission-critical enterprise-wide applications. The IBM System z9TM, the latest generation of the IBM distinguished family of mainframe systems, has come a long way from its IBM System/360TM heritage. Likewise, its z/OS® operating system is far superior to its predecessors, providing, among many other capabilities, world-class, state-of-the-art support for the TCP/IP Internet protocol suite. TCP/IP is a large and evolving collection of communication protocols managed by the Internet Engineering Task Force (IETF), an open, volunteer organization. Because of its openness, the TCP/IP protocol suite has become the foundation for the set of technologies that form the basis of the Internet. The convergence of IBM mainframe capabilities with Internet technology, connectivity, and standards (particularly TCP/IP) is dramatically changing the face of information technology and driving requirements for ever more secure, scalable, and highly available mainframe TCP/IP implementations. This new and improved Communications Server (CS) for z/OS TCP/IP Implementation series provides easy-to-understand step-by-step how-to guidance on enabling the most commonly used and important functions of CS for z/OS TCP/IP. In this IBM RedbookTM we begin with a discussion of Virtual IP Addresing (VIPA), a TCP/IP high-availability approach introduced by the z/OS Communications Server. We then proceed to show how VIPA can be used for high availability--both with and without using a dynamic routing protocol. Then we discuss a number of different workload balancing approaches that