What Is a Network Layer Model?

Internet communication does not work through one mechanism alone.

When you view a website, your device combines processes such as carrying communication over Wi-Fi or wired LAN, delivering it to a destination using IP addresses, managing how communication is delivered with TCP or UDP, and letting applications communicate with each other through HTTP or DNS.

If you try to understand all of these at the same level, the overall picture of network communication becomes hard to see.

That is why the perspective of dividing communication into layers is important.

A network layer model is a way of organizing communication mechanisms by role. By thinking of each layer as responsible for a different role, complex communication becomes easier to understand.

This article looks at the layered structure of networks, focusing on the TCP/IP model, which is often used when understanding Internet communication.

The Four Layers of the TCP/IP Model

In the TCP/IP model, communication is broadly considered through the following four layers.

Looking at these four layers makes it easier to organize the roles in network communication.

The important point is not to memorize every detail. First, understand the differences between the roles each layer handles.

This article aims to help you understand the overall TCP/IP model. The detailed mechanisms of IP addresses, routing, TCP, UDP, HTTP, HTTPS, DNS, and similar topics are covered in separate articles.

Here, instead of explaining each technology in detail, we will organize "which layer of the TCP/IP model it relates to" and "what role it has in communication as a whole."

Why Think in Layers?

The reason to think in layers is to understand communication by separating its roles.

For example, even when you cannot access a website, there is not always only one possible cause.

There may be a problem with the Wi-Fi or wired LAN connection. There may be a problem reaching the communication destination using an IP address. The TCP connection may not be established correctly. DNS may be unable to convert the domain name into an IP address. Or an error may simply be returned as an HTTP response.

Even with the same state of "communication is not possible," the layer where the problem occurs can differ.

Using the TCP/IP model, you can think about communication problems in the following way.

In this way, thinking in layers lets you organize network mechanisms part by part.

Network Interface Layer

The lowest layer in the TCP/IP model is the network interface layer.

This layer is the part that actually carries communication. Wired LAN, Wi-Fi, optical fiber, mobile communication, and similar technologies relate to it.

Communication data does not simply move through empty space as-is. In practice, it is carried as electrical signals, radio waves, optical signals, and similar forms.

For example, wired LAN uses cables for communication. Wi-Fi uses radio waves for communication. Communication using optical fiber uses optical signals.

The role of this layer is not to understand HTTP content or DNS query content. Its first role is to make it possible to actually carry signals.

If the network interface layer is not functioning, IP communication, TCP communication, and HTTP communication above it cannot be established either.

Depending on the material, this layer may also be called the "network access layer" or "link layer." The names differ, but here it is enough to understand it as "the layer for actually carrying communication."

Internet Layer

Even if the network interface layer makes it possible to carry signals, that alone does not deliver communication to a destination on the Internet.

The next thing needed is a mechanism for deciding "where to deliver it." The Internet layer has this role.

The center of the Internet layer is IP.

IP handles the source and destination of communication using IP addresses. When your device accesses a website, communication is sent toward the other party's IP address.

There are many routers and network devices on the Internet. Communication data is delivered to the intended destination while passing through them.

At this layer, "which party to deliver to" is important.

The important point here is that IP does not understand the communication content itself. IP does not judge HTTP content or the contents of a web page; it handles the information needed to deliver communication to the destination.

In other words, the Internet layer is the layer responsible for "where communication should be directed."

Transport Layer

Even with a mechanism for delivering communication to a destination through the Internet layer, how to deliver the data is a separate problem.

The transport layer has this role.

The transport layer uses TCP and UDP.

TCP is a mechanism for sending and receiving data while managing a connection with the communication partner. It relates to whether data arrives in order and how to handle data that is lost along the way.

UDP is a simpler communication method than TCP. Because it does not perform connection management or delivery confirmation as strictly as TCP, it is sometimes used for communication where real-time behavior is important.

This layer relates to points such as "how to deliver the data," "whether to manage a connection," and "which application to pass it to."

For example, even on the same device, multiple applications such as a browser, chat app, game, and mail app may be communicating at the same time. At the transport layer, port numbers and similar information are used to handle which communication should be passed to which application.

In other words, the transport layer is the layer responsible for "which method to use to deliver communication headed for the destination."

Application Layer

The top layer of the TCP/IP model is the application layer.

This layer handles the rules by which applications exchange information with each other.

When you view a website, HTTP or HTTPS is used. HTTP is a rule for browsers and web servers to exchange information.

For example, the browser requests "I want to retrieve this page" from the web server, and the server returns HTML, images, CSS, JavaScript, and similar data.

DNS also relates to the application layer. DNS is a mechanism for converting domain names into IP addresses.

When you enter a domain name in a browser, the device looks up the IP address corresponding to that name. It then uses that result to connect to the actual communication destination.

At this layer, information is exchanged in a form applications can understand. With HTTP, communication happens in the form of requests and responses. With DNS, a query asks for the IP address corresponding to a domain name.

In other words, the application layer is the layer responsible for "what meaningful information applications exchange with each other."

Viewing Web Access Through the TCP/IP Model

When you access a website, each layer of the TCP/IP model works together.

However, actual communication does not "start from the bottom layer in order." When you access a website, processing begins with an application such as a browser.

On the sending side, communication is passed from the upper layers to the lower layers.

First, at the application layer, the browser creates an HTTP or HTTPS request. If needed, DNS also looks up the IP address corresponding to the domain name.

Next, at the transport layer, TCP or UDP manages how data is delivered. Many web communications use TCP, but some communication, such as HTTP/3, uses UDP.

Next, at the Internet layer, information for heading toward the communication destination is handled using IP addresses. Here, the communication data is sent toward the destination IP address.

Finally, at the network interface layer, communication is carried as actual signals through Wi-Fi, wired LAN, optical fiber, mobile communication, and similar paths.

In this way, on the sending side, data created in the upper layers is passed down to lower layers while information needed for communication is added. This process is called encapsulation.

Communication that reaches the server side is then passed from the lower layers to the upper layers.

Signals are received at the network interface layer, destination information is handled at the Internet layer, the delivery method is processed at the transport layer, and the communication is finally passed to the web server application.

In other words, web access flows as follows.

Seen this way, the TCP/IP model is not simply a story of "communicating in order from the bottom." On the sending side, processing goes from top to bottom, and on the receiving side, from bottom to top, allowing applications to communicate with each other.

Accessing a website is ultimately an exchange between applications: a browser and a web server. To make that exchange work, the transport layer, Internet layer, and network interface layer support it from below.

Difference From the OSI Reference Model

The OSI reference model is also often used as a network layer model.

The OSI reference model divides communication into seven layers. The TCP/IP model, by contrast, is a four-layer model used more practically for understanding Internet communication.

At the beginner stage, you do not need to memorize the seven layers of the OSI reference model in detail. It is easier to start by understanding the TCP/IP model, which connects more directly to real Internet communication.

The OSI reference model is also an important way of thinking for understanding networks. If you want to learn in more detail, it is useful to look up the OSI reference model as well.

This article organizes the basic structure of communication mainly through the TCP/IP model, which is easier to connect to actual Internet communication.

Important Perspectives for Thinking About Anonymity

The TCP/IP model is also important when thinking about anonymity.

That is because different information is visible at different layers in communication.

For example, IP addresses relate to the Internet layer. TCP and UDP port numbers relate to the transport layer. DNS, HTTP, cookies, User-Agent, and similar information relate to the application layer.

All of these are information related to communication, but they are not all in the same layer.

Even if you hide an IP address, cookies may make it possible to judge that the same user is involved. Even if communication content is encrypted with HTTPS, not all information about the destination disappears. If DNS is handled improperly, it may be possible to see which domain you tried to access.

In other words, when thinking about anonymity, you need to separate "what you want to hide" from "which layer's information is visible."

Understanding the TCP/IP model makes it easier to organize which part of communication IP addresses, DNS, HTTPS, cookies, User-Agent, and similar items relate to.

Summary

A network layer model is a way of thinking for understanding communication by dividing it by role.

This article focused on the TCP/IP model, which makes Internet communication easier to understand.

In the TCP/IP model, communication is considered through the following four layers.

Communication is established through the cooperation of these layers.

The network interface layer alone cannot decide the destination. The Internet layer alone cannot manage how data is delivered. The transport layer alone cannot handle retrieving web pages or resolving DNS names. The application layer alone cannot physically carry communication.

Internet communication works because each layer has a different role.

Understanding the TCP/IP model makes it easier to organize the overall picture of network communication. It also makes it easier, when thinking about anonymity, to judge which layer's information is visible, what is hidden, and what is not hidden.

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