This chapter discussed the building blocks for internetworking design and the key components of the software and protocols running over such networks. We can summarize these as follows:
Network design is an iterative process of continuous refinement. A good network design is logical and consistent and should deliver acceptable performance and cost metrics.
IP is the dominant protocol suite today. Its role is now central to all Internet application delivery. IP protocols and services are independent of the underlying network hardware. Each host on an IP network should have a unique IP address, which universally identifies the device. IPv4 addresses are becoming depleted. A new version of IP, IPv6, is being introduced to provide scalability for the next-generation Internet.
Networks can be constructed from a wide variety of local and wide area media interconnected using devices such as repeaters, bridges, switches, routers, and gateways. All of these devices have different uses depending upon the scale of the network and the protocols in operation. The boundary between devices such as switches, routers, and bridges is becoming blurred. Current trends indicate that further integration and greater performance are likely as these technologies mature.
LAN switches are best used to provide simple connectivity for local workgroups, server clusters, and LAN backbone applications. Bridges can be used for similar applications, but they are dying out, largely replaced by higher-speed switches (unless they are required for specific applications such as bridging between different media technologies). Switches and bridges are useful on sites where there is limited technical and management support available and are best suited for environments where it is undesirable or impractical to configure end systems for operation with routers. Bridges and switches may also be the only option where some of the protocols in use are nonroutable (e.g., DEC LAT or IBM SNA).
Hybrid multiprotocol routers have largely replaced conventional bridges and routers in the marketplace; this is because they combine the functions and hence the benefits of dedicated bridges and routers. Subject to costs, they are suitable as standard all-purpose intermediate nodes for most internetwork requirements. Routers provide superior traffic management by enforcing hierarchical addressing and least-cost routing to optimize expensive wide area bandwidth.
 T. Kenyon, High-Performance Network Design: Design Techniques and Tools (Woburn, MA: Digital Press, 2001).
 www.itu.ch, the International Telecommunications Union (ITU) home page
 U. Black, TCP/IP and Related Protocols, (New York: McGraw-Hill, 1992).
 D. E. Comer, Internetworking with TCP/IP, Vol. I: Principles, Protocols, and Architecture (Englewood Cliffs, NJ: Prentice Hall, 1991).
 D. E. Comer, Internetworking with TCP/IP, Vol. II: Design, Implementation, and Internals, (Englewood Cliffs, NJ: Prentice Hall, 1991).
 P. Miller, TCP/IP Explained, (Woburn, MA: Digital Press, 1997).
 W. Stallings, Data and Computer Communications, (Englewood Cliffs, NJ: Prentice Hall, 1997).
 D. Crocker, Standard for the Format of ARPA Internet Text Messages, RFC 822, August 13, 1982.