What Is the Internet?

Understanding what the Internet is makes every subsequent web concept — HTTP, DNS, servers — click into place. This page builds the mental model from the ground up, starting with history and finishing with a practical walkthrough.

Target reader: Anyone who uses the Internet but has never studied how it works. Estimated time: 10 min read + 5 min hands-on Prerequisites: None

What the Internet Is

The Internet is a global network of computers connected to each other so that they can exchange data. The key word is “network of networks”: individual local networks (a home Wi-Fi, a university campus network, a corporate intranet) are all linked together into one massive, planet-wide system.

A useful analogy is the postal system. Each house has a unique address. Letters can travel from any house to any other house — even across countries — because postal networks around the world agree on common rules for routing mail. The Internet works the same way, but for data.

A Brief History

The Internet grew out of ARPANET, a US Department of Defense research project launched in 1969. Its original goal was to let research computers at different universities share data. The breakthrough insight was a technique called packet switching (more on this below), which made the network far more resilient than earlier circuit-switched telephone networks.

Key milestones:

Year	Event
1969	ARPANET connects 4 university computers
1983	TCP/IP adopted as the standard protocol
1991	Tim Berners-Lee invents the World Wide Web (HTTP + HTML)
1993	The first graphical web browser (Mosaic) is released
2000s	Broadband, smartphones, and the social web
2010s	Cloud computing, streaming, and mobile-first design

The Internet and the Web are often used interchangeably, but they are different things. The Internet is the underlying infrastructure (cables, routers, protocols). The Web is one service that runs on top of that infrastructure — the system of linked pages accessed via HTTP.

What an IP Address Is

Every device on the Internet needs a unique identifier so that data can find its way to the right destination. That identifier is an IP address (Internet Protocol address).

Think of it like a home address. A street address tells the postal system exactly where to deliver a letter. An IP address tells the Internet exactly where to deliver data.

IPv4 example:  192.168.1.1
IPv6 example:  2001:0db8:85a3:0000:0000:8a2e:0370:7334

IPv4 uses four numbers between 0 and 255, giving about 4.3 billion unique addresses. Because the world ran out of IPv4 addresses, IPv6 was introduced with a vastly larger address space (about 340 undecillion addresses).

A domain name like example.com is a human-friendly alias for an IP address. The DNS system (covered in a later page) translates between the two.

Packet Switching

When data travels across the Internet, it is not sent as one continuous stream. Instead, it is broken into small chunks called packets.

Imagine sending a long letter, but the envelope can only hold one page. So the letter is split into pages, each page is numbered, and each page is mailed separately. They might travel via different routes, arrive out of order, and need to be reassembled at the destination. Packet switching works exactly like this.

Original file (1,000 KB)
        |
        v
  +----------+  +----------+  +----------+
  | Packet 1 |  | Packet 2 |  | Packet 3 |  ...
  | (1–250KB)|  |(251–500KB)|  |(501–750KB)|
  +----------+  +----------+  +----------+
        |              |              |
        v              v              v
   Route A          Route B        Route A
        \              |              /
         \             v             /
          +-----> Destination <-----+
                       |
                       v
              Reassembled file

The advantages of this approach:

• Resilience: If one route fails, packets can take alternative paths.
• Efficiency: Multiple conversations can share the same physical links simultaneously.
• Scalability: No dedicated connection needs to be held open for the entire duration of a transfer.

The TCP/IP Protocol

Protocols are agreed-upon rules. For the Internet to work, every device must follow the same rules for sending and receiving data. The dominant protocol suite is TCP/IP.

It has two layers:

Protocol	Role
IP (Internet Protocol)	Handles addressing and routing. Decides which path each packet takes.
TCP (Transmission Control Protocol)	Ensures reliable delivery. Checks that all packets arrived, requests retransmission of lost packets, and reassembles them in the right order.

A practical analogy: IP is the postal worker who routes the envelope, and TCP is the certified-mail system that requires a signature and resends the letter if it never arrives.

There is also UDP (User Datagram Protocol), which skips the reliability checks for speed. Video streaming and online games often use UDP because a slightly dropped frame is better than waiting for a retransmission.

Practical Example: What Happens When a URL Is Typed

Here is a high-level overview of the journey (each step is covered in more detail in the following pages):

1. A URL like https://example.com/about is typed into a browser.
2. The browser asks a DNS server to translate example.com into an IP address.
3. The browser opens a TCP connection to that IP address.
4. The browser sends an HTTP request asking for /about.
5. The server sends back an HTTP response containing HTML.
6. The browser parses and renders the HTML to display the page.

The entire process typically takes less than a second, involving dozens of routers passing packets across continents.

Hands-On: Finding Your IP Address

To see the IP address assigned to a computer, open a terminal and run:

# macOS / Linux
curl ifconfig.me

This contacts a public service that reflects the public IP address back. The result will be the IP address that web servers on the Internet see when receiving a request from that machine.

Summary

• The Internet is a global network of networks that exchange data using shared protocols.
• Every device has a unique IP address (like a postal address).
• Data travels in small chunks called packets, which can take different routes and are reassembled at the destination.
• TCP/IP is the standard protocol suite that makes reliable data exchange possible.

FAQ

Q: Is the Internet the same as the Web?

A: No. The Internet is the physical and logical infrastructure — cables, routers, and protocols. The Web (World Wide Web) is one application that runs on the Internet, consisting of linked HTML pages accessed via HTTP. Email, SSH, and FTP are other applications that also run on the Internet but are not part of the Web.

Q: What is the difference between IPv4 and IPv6?

A: IPv4 uses 32-bit addresses (about 4.3 billion unique values), which have been exhausted by the growth of the Internet. IPv6 uses 128-bit addresses, providing a near-unlimited number. Most devices now support both, and the transition from IPv4 to IPv6 is ongoing.

Q: What is a local IP address vs a public IP address?

A: A local IP address (e.g., 192.168.x.x) is assigned by a router and is only visible within a home or office network. A public IP address is assigned by an internet service provider and is what the outside world sees. A device behind a home router typically shares one public IP with all other devices on that network.

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Next: How the Web Works

Link to this page (Japanese): インターネットとは何か