By the beginning of 2003 more than 170 million computers (called hosts, in Internet terminology) were connected to the Internet, according to research by the Internet Software Consortium, in Redwood City, Calif.
The Internet (or, informally, the Net) has neither an owner nor a single individual, organization, or governmental authority controlling its contents or governing access to it. Rather, it comprises many smaller computer networks that have been linked together; costs associated with the system are paid voluntarily by those who use and maintain it. Users typically access the Internet through a computer equipped with the software and hardware necessary to permit data to be sent to and received from other computers. An Internet connection is typically established by linking the computer through a telephone or cable line or by way of wireless technology, to an Internet service provider (ISP), a company whose computers are in turn linked to the Internet's hierarchy of computer networks. Host computers, such as those of an ISP, that provide connection to the Internet or other services are known as servers. Hosts receiving those services are called clients. In many cases a computer both gives and receives services, making it a server as well as a client.
History
The Internet's origins date back to the early 1960s, when the U.S. government began considering the building of a communications system capable of withstanding large-scale disruptions, even in the event of a nuclear war. It was reasoned that the system should have no central control or authority, so that were any single part of it destroyed, the rest of the system could continue functioning. Moreover, it was agreed that the system should be designed so that even if large portions of it were knocked out of service, the remnants could keep working and that any damaged sections, once repaired, could be quickly returned to service. Ideas formed in response to these requirements focused primarily on packet switching (breaking an electronic data file into smaller units, or packets, and transmitting them separately over the system) and the automatic routing of data.
An experimental packet-switching network went into operation in the fall of 1969, connecting computers at four institutions: the University of California at Los Angeles, the University of California at Santa Barbara, the Stanford Research Institute, and the University of Utah in Salt Lake City. The system was called the ARPANET, named after the United States Department of Defense's Advanced Research Projects Agency, which funded the experiment.
By 1972 the ARPANET comprised 37 host computers spread across the United States. While the network's original purpose was to allow remote use of the few supercomputers then in existence, most of the traffic on the ARPANET soon consisted of electronic mail (e-mail) sent person-to-person or to groups of users belonging to automated mailing lists. The system came to be used therefore not only as a means of scientific collaboration but also as a facility for the discussion of topics of technical and professional concern (for example, computer network design) as well as subjects of more personal interest (such as science fiction). This continued to be the case as the ARPANET expanded through the 1970s, reaching 213 hosts by 1981.
The ARPANET initially used a communication protocol (the set of rules employed by computers when transmitting information back and forth) called the network control program (NCP). This was replaced in 1982 with the more advanced TCP/IP, actually a combination of two protocols: the Internet protocol (IP) and the transmission control protocol (TCP). The IP is used to deliver the data packets that together make up a message file, carrying them from the sender's computer to their destination across a network. Once the packets reach the intended computer, the TCP reassembles them into the complete message file. The introduction of the TCP/IP allowed the ARPANET to accommodate the seamless interconnection of many different computer networks despite variations between them in size, speed, and technology.
The military elements targeted by the original ARPANET design separated from the network in 1983, forming MILNET. In 1986 the U.S. National Science Foundation (NSF) launched its own network based on ARPANET technology. Connected to the ARPANET, the NSFNET was meant to enhance communication within the academic and scientific community.
Although during the 1980s rising personal computer use triggered the rapid growth of the ARPANET, in 1990 the network ceased to exist, having been officially decommissioned and replaced by a much larger one. This new network, the Internet, had evolved from a combination of a series of networks, including the NSFNET. (The Internet took its name from the previously mentioned Internet protocol.)
IP Address
Every computer on the Internet is assigned a unique number, an IP address, that enables it to receive data using the TCP/IP. When packets making up a computer file are transmitted across the Internet, each packet contains the intended recipient's (as well as the sender's) IP address, allowing the packet to find its destination.
