f the future. The original network was developed in the early 1960s by the U.S. gov- ernment based on the Defense Advanced Research Project Agency Network (ARPANET). The U.S. Department of Defense (DOD) funded the project in 1969. The purpose of the ARPANET was to provide the existing U.S. Defense Department Network (DDN) with a digital highway between its sites and the Pentagon. This high-speed highway was designed to survive a nuclear holocaust or first strike. The data on this network included datagrams (intelligent data packets) to automatically route themselves around a failed segment (link) or nodes on the network. Protocols and Technologies Enabled the Internet During the 1970s, the ARPANET gradually transformed into a true Inter- net as new protocols and technologies became available, and as defense, research, and scientific organizations were added to the network. While these various entities proliferated, the need for robust and secure network partitions (subnetworks) grew. This need led to the development of a spe- cific protocol for transmitting datagrams between the various subnetworks. The creation of TCP/IP protocols facilitated the connection of devices called routers, reliable gateways, and other switching devices. The early routers developed by BBN were designed to fit into the Digital Equipment Corpo- ration (DEC) Packet Data Protocol (PDP) minicomputer architecture used by academic and research institutions. In subsequent developments, ARPANET packet technology evolved into the X.25 packet-switching prod- uct suite, enabling Internet-designed topologies of commercial and govern- ment networks in the 1980s through the 1990s. What Then Is the Internet? The easiest way to describe this Internet is that it consists of many inter- connected networks, the pieces owned and operated by different operators, linked together. The Internet is not a single entity, but rather is a group of Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGr