TCP IP Model

In previous courses, we looked at how early networks used models from vendors like IBM and DEC. These models worked only with their own equipment, which made interoperability impossible. To fix this, standardized models were needed.

The OSI Model helps us grasp how communication occurs in stages.
But in practice, the model that actually runs the Internet is the TCP/IP Model.

Figure 1 – TCP/IP model: 4 layers with example protocols.

TCP/IP isn't a reference like OSI. It's a real implementation. Your devices use their protocols daily to communicate across networks.

Before we explore the TCP/IP layers, let’s look at how protocols are defined and who sets their rules.

Who defines the rules?

The Internet Engineering Task Force (IETF) is a global community that coordinates the development of Internet standards. It does not “write” every protocol itself. Instead, engineers, researchers, and companies from all over the world form working groups.

Figure 2 – IETF (Internet Engineering Task Force) logo

They propose ideas, discuss them, and refine them until there is a consensus. Once a proposal receives acceptance, the team publishes it as an RFC (Request for Comments).

What is an RFC?

An RFC is a document that explains in detail how a protocol should work.
Think of it as a guide that ensures all devices “speak the same language.”

Key facts about RFCs:

• Each RFC has a unique number and is free to read online.

• Some are informational or experimental, while others become official Internet standards.

• RFCs ensure protocols like TCP, IP, DNS, and HTTPS behave the same way across all devices and vendors.

Examples of RFCs

• RFC 791 – IPv4: Defines Internet Protocol version 4.

• RFC 793 – TCP: Explains how TCP provides reliable communication.

• RFC 1149 – IP over Avian Carriers: A humorous RFC describing data transfer using pigeons.🐦

Thanks to RFCs, your laptop, phone, and routers from different brands can all communicate smoothly.

Now that you know how experts standardize protocols, let’s explore the four layers of the TCP/IP Model in detail.

Now that you understand how RFCs standardize protocols, let's see how they fit into the TCP/IP Model.

From proprietary models to TCP/IP

The TCP/IP Model, also called the Internet Protocol Suite, is the real backbone of modern networking.
It was developed by the U.S. Department of Defense (DoD) in the 1970s and became widely adopted in 1983.

Unlike the OSI Model, which is mainly theoretical, the TCP/IP Model is practical and implemented. It defines the protocols your devices use every day to communicate.

The Four Layers of TCP/IP

The TCP/IP Model groups communication into four main layers:

1. Application

2. Transport

3. Internet

4. Network Access

TCP/IP vs OSI

The diagram below shows how the TCP/IP layers map to the OSI Model, with examples of protocols at each level.

Figure 3 – TCP/IP correspondence → OSI

Keep in mind: OSI is a reference model, while TCP/IP is the real implementation that makes networks work.

Application Layer

The Application Layer combines OSI’s Application, Presentation, and Session layers.
Users interact with the network through services. These include web browsing, sending emails, and transferring files.

Figure 4 – TCP/IP Application Layer

Examples of protocols:

• HTTP/HTTPS → Web Browsing

• SMTP/IMAP/POP3 → Email

• DNS → Domain Name Resolution

• FTP → File Transfers

Some of these may be new to you; that’s fine. We’ll cover them step by step later in the course.

Transport Layer

The Transport Layer (equal to OSI Layer 4) provides end-to-end delivery between hosts.
It divides data into segments, manages connections, and reassembles streams at the destination.

Figure 5 – TCP/IP Transport Layer

Key protocols:

• TCP: reliable, connection-oriented

• UDP: faster, connectionless, and used in real-time apps

The Transport Layer also uses port numbers to identify services. For example, HTTPS runs on port 443, so when you visit https://pingmynetwork.com, your browser and the server know how to communicate securely.

Internet Layer

The Internet Layer (like OSI Layer 3) ensures data can travel across different networks.
Routers operate here, forwarding packets based on IP addresses.

Figure 6 – TCP/IP Internet Layer

Main functions:

• IP addressing (IPv4/IPv6) to identify hosts

• Routing packets through routers

• Diagnostics with ICMP (ping, traceroute)

• Address resolution with ARP (IP → MAC)

If ICMP or ARP sounds new, don’t worry; we’ll study them in detail later.

The Internet Layer’s job is simple but vital: it makes sure your data takes the right path across networks.

Network Access Layer

The Network Access Layer merges OSI’s Data Link and Physical layers.
This is where data actually leaves your device and travels across the network.

Figure 7 – TCP/IP Network Access Layer

Responsibilities:

• Framing with MAC addresses and error checking (FCS)

• Media access control (Ethernet CSMA/CD, Wi-Fi CSMA/CA)

• Bit transmission as electrical signals (in cables) or as radio waves (in wireless)

Consider this: whether you connect your laptop with a cable or via Wi-Fi, this layer ensures your data moves from one device to another.

To better understand how the TCP/IP layers work together, let’s follow the path of data through a simple network:

• A PC is connected to Switch 1

• Switch 1 is connected to Router R1

• R1 is connected to Switch 2

• Switch 2 is connected to a Server

Figure 7 – TCP/IP Data Flow

Step 1 – From the PC to Switch 1

The PC sends a message. The switch looks at the physical address (MAC address) inside the frame and decides which port to forward it to.
Switches only care about local delivery inside the same network.

Step 2 – From Switch 1 to Router R1

When the message reaches the router, things change. The router looks at the IP address and decides the best path to reach the server’s network.
Routers make communication possible between different networks.

Step 3 – From Router R1 to Switch 2 and the Server

The router forwards the data toward Switch 2.
Switch 2 again uses the MAC address to deliver the frame directly to the server.
When the server receives it, the data travels up its TCP/IP stack until it reaches the application (for example, a web server that sends back a page).

The TCP/IP Model is not theory; it’s the real foundation of the Internet.
Every time you browse a website, send a message, or stream a video, these four layers are at work.

Key takeaways from this course

• Protocols are standardized through RFCs, ensuring that devices from different vendors can communicate.

• TCP/IP has four layers:

  • Application → user interaction (web, email, DNS)

  • Transport → reliable or fast delivery using port numbers (TCP/UDP)

  • Internet → logical addressing and routing with IP addresses

  • Network Access → local delivery using MAC addresses and physical signals

By now, you understand how data travels from your computer to any server across the Internet.

What’s Next?

Theory is useful, but let’s make it more practical.
We’ve seen that the Internet Layer uses IP addresses, and the Network Access Layer uses MAC addresses.

But how does a device actually find the MAC address that corresponds to an IP address?

That’s where the ARP protocol (Address Resolution Protocol) comes in.
In the next course, we’ll explore ARP step by step with examples.