OSI and TCP/IP Models

Why layered models exist

Layered models divide networking work into smaller responsibilities. You already know about hosts, network devices, MAC addresses, and IP addresses. The models give those ideas a place on one map.

Imagine sending a short message from a laptop to another host. One part of the system creates the message, another supplies delivery information, and another carries signals across a cable or wireless link. Separating those jobs lets us study one responsibility at a time.

A protocol is an agreed set of communication rules. A model groups responsibilities; it is not itself a protocol. Neither model is a physical stack of seven or four devices. A host's software and network interface work together, and real products can combine responsibilities.

For now, aim to explain each layer's main job rather than memorize every protocol or header field.

The seven OSI layers

OSI means Open Systems Interconnection. Its seven conceptual layers are numbered from Physical (1) to Application (7). The table is shown from the top down, as data is prepared for sending.

The Application layer is not the entire browser or phone app: it describes the network-service responsibility used by that app. Session and Presentation describe useful jobs, but modern Internet software does not always implement them as separate layers.

Devices are useful reminders, not rigid labels. A hub repeats signals; an ordinary Ethernet switch works with frames; a router connects IP networks. A host uses the whole stack, and one physical product can contain several functions.

The TCP/IP model

TCP/IP stands for Transmission Control Protocol / Internet Protocol. The name also refers to the Internet protocol suite, a family of protocols used together. It does not mean every exchange must use TCP: UDP is another transport protocol in that suite.

We use the common four-layer TCP/IP model:

• Application: services and the application's data exchange, such as HTTP or DNS.
• Transport: communication between application processes, using TCP or UDP.
• Internet: IP addressing and delivery across networks.
• Network Access: local-link frames and the signals that carry them.

Network Access is also called the Link layer. Some teaching resources split it into Data Link and Physical to make a five-layer model. That is a different grouping, not a different Internet. This course's comparison uses four TCP/IP layers consistently.

OSI-to-TCP/IP mapping

Read across the diagram to compare responsibilities, rather than comparing layer numbers. OSI has seven layers while this TCP/IP model has four, so their numbers are not interchangeable.

TCP/IP Network Access roughly combines OSI Physical and Data Link. TCP/IP Internet aligns with OSI Network. Transport aligns with Transport. TCP/IP Application groups the OSI Session, Presentation, and Application responsibilities. These are helpful conceptual mappings, not a requirement that every protocol fit perfectly into one box.

Encapsulation and layer matching

Encapsulation means adding delivery information around data as it moves down the sender's stack. Decapsulation means examining and removing that information as the receiver passes the data upward.

Think of putting a message inside labeled envelopes. The labels help deliver the message; they do not replace it. In this simplified TCP example, the names change as delivery information is added:

Application data → TCP segment → IP packet → Ethernet frame → bits carried as signals.

A header is delivery information placed before the data it carries. Ethernet also uses information at the end, called a trailer. A protocol data unit (PDU) is simply the name for the unit of data at a particular layer. You do not need to learn detailed header fields yet. With UDP, the transport unit is called a datagram rather than a TCP segment.

The player shows the sender going down the stack and the receiver going up. It is one simplified transfer, not the complete process of opening a website. The top three OSI responsibilities are grouped into the Application step; no separate Session or Presentation header is assumed.

Use Pause, Next, Previous, and Restart to compare the original application data with the delivery information around it. Watch the active layer and PDU change together. Detailed protocol mechanics belong in the later layer modules.

Now match four familiar examples to their TCP/IP groups. You can retry an answer or move between examples; this practice is not an additional completion requirement.

Knowledge check

How we will use this map: after Computer Network Basics, the course restructure follows Physical → Data Link → Network → Transport → Application, Session & Presentation. We will start with the signals and build upward. A separate later module will connect those ideas into how hosts communicate.

For this introduction, remember the jobs: signals, local frames, IP delivery, application transport, and the application's exchange. The OSI model separates the upper responsibilities; TCP/IP groups them. Both help you organize the same networking concepts.

References: OSI reference model (ITU-T X.200) and Internet protocol layers (RFC 1122, section 1.1.3).