The Internet can be traced back to several early networks. In 1969, the US Department of Defense's Advanced
Research Projects Agency Network connected computers at four universities, forming ARPANet. Other early networks, including NSFNet, BITNET, and CERN, similarly connected academics and research facilities. By 1990, these networks had become connected and were using the same transmission control protocol/internet protocol (TCP/IP).
The modern Internet is a global network of computers that communicate using standard protocols (Figure 2). Within organizations and across distributed projects such as NEON (National Ecological Observatory Network) may be networks that are not publicly accessible. Generally, networked computers have connections both to their local or private networks, intranets, and to the global network, the Internet. Both intranets and the Internet can be used by people to exchange information such as electronic mail, or email, or for exchanging files using the file transfer protocol (FTP). When the information crosses from an intranet to the wider Internet, it goes through a gateway. Firewalls may be set up at any point in these networks to monitor traffic so as to detect and prevent unauthorized or malicious activity. Firewalls may consist of specialized hardware devices or software applications.
The primary model of Internet communication is using a software application, a client, which communicates either directly with other clients or more typically with servers. Servers are configured to deliver information when a client requests it. Each computer, whether running clients or acting as servers, has an IP address. An IP address is numerical and can be associated with readable domain names. TCP/IP organizes and routes information between computers using their IP addresses much as people use street addresses to send letters and packages.
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