> For the complete documentation index, see [llms.txt](https://azunyan.gitbook.io/internet-protocol/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://azunyan.gitbook.io/internet-protocol/1.-computer-network-and-the-internet/1.1-what-is-the-internet.md).

# 1.1 What is the internet?

#### 1.1.1 A Nuts-and-Bolts description

The Internet is a computer network that interconnects billions of computing devices throughout the world.

In Internet jargon, all of these devices (TVs, gaming consoles, thermostats, home security systems, home appliances, watches, eye glasses, cars, traffic control systems, and more) are called **hosts** or **end systems**.

End systems are connected together by a network of **communication links** and **packet switches**.

Different links can transmit data at different rates, with the **transmission rate** of a link measured in bits/second.

When one end system has data to send to another end system, the sending end system segments the data and adds header bytes to each segment. The resulting packages of information, known as **packets** in the jargon of computer networks, are then sent through the network to the destination end system, where they are reassembled into the original data.

A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links. Packet switches come in many shapes and flavors, but the two most prominent types in today’s Internet are **routers** and **link-layer switches**.

The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system is known as a **route** or **path** through the network.

End systems access the Internet through **Internet Service Providers (ISPs).** Each ISP is in itself a network of packet switches and communication links.

Lower tier ISPs are thus interconnected through national and international upper-tier ISPs and these upper-tier ISPs are connected directly to each other.

End systems, packet switches, and other pieces of the Internet run protocols that control the sending and receiving of information within the Internet. The **Transmission Control Protocol (TCP)** and the **Internet Protocol (IP)** are two of the most important protocols in the Internet. The IP protocol specifies the format of the packets that are sent and received among routers and end systems. The Internet’s principal protocols are collectively known as **TCP/IP**.

#### 1.1.2 A services description

The applications are said to be distributed applications, if they involve multiple end systems that exchange data with each other.

Internet applications run on end systems—they do not run in the packet switches in the network core.

End systems attached to the Internet provide a **socket interface** that specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system.

#### 1.1.3 What is a protocol?

A **protocol** defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.


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