The Internet is a network of computer networks that uses the same computer networking language (or protocol), TCP/IP. Before the wide adoption of TCP/IP as a common language that most computer systems understood, many computer networks used proprietary protocols owned by specific corporations.
In practical terms, this meant that you could only communicate with the other people who used the same proprietary network. Whether this was IBM’s TokenRing or AppleShare at the office, or Compuserve at home, you could only reach, at most, other people or network resources (like printers, email servers, or shared databases) that used the same company’s networking products.
Since no one company owns the TCP/IP protocol, or any of the other protocols in the Internet protocol suite, any computer or software company can use these protocols to connect their devices, programs, or products to the rest of the world. You use some of these protocols every day, including HTTP (for browsing the World Wide Web) or SMTP, POP and IMAP (for sending and receiving email).
Notice that the World Wide Web — the web sites you visit using a browser on your computer, tablet or smartphone — is just one part of the Internet. Also, the Internet can operate over all kinds of connections, including wired (i.e., Cat 5 or fiber optic cable) or wireless (i.e., WiFi, Bluetooth, and cellular) networks.
Because of this flexibility and openness, the Internet has grown to become the largest shared network in history, much larger than the original (landline) telephone network or any private computer network.
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The Internet is the global system of interconnected computer networks that use the Internet protocol suite (TCP/IP) to link devices worldwide. It is a network of networks that consists of private, public, academic, business, and government networks of local to global scope, linked by a broad array of electronic, wireless, and optical networking technologies.