what is TCP/IP and OSI? // FREE CCNA // EP 3
Based on NetworkChuck's video on YouTube. If you like this content, support the original creators by watching, liking and subscribing to their content.
Networking interoperability required shared standards after proprietary networks prevented devices from communicating across vendors.
Briefing
Networking became possible only after engineers agreed on shared rules for how devices should communicate—rules that later crystallized into the TCP/IP and OSI models. The practical payoff is obvious today: an iPhone can exchange photos with a Raspberry Pi, and computers from different companies can interoperate. That wasn’t always the case. Early networks were often proprietary, meaning IBM-style networking didn’t reliably talk to Apple- or Microsoft-style networking, because each vendor effectively “spoke a different language.” The TCP/IP and OSI models emerged to standardize those languages so devices could communicate across brands and networks.
The story traces back to ARPANET in 1969, funded by the U.S. Department of Defense, which helped popularize packet switching—the core idea behind sending data in chunks across a network. But the early challenge wasn’t just inventing networking; it was making different computers communicate in a consistent way. As companies like IBM built their own networking approaches, incompatibility became the norm. Without common standards, devices couldn’t reliably exchange data, much like trying to use the wrong charging cable across ecosystems.
Eventually, the industry converged on networking standards and layered models that break communication into manageable functions. TCP/IP is the model most widely implemented in real systems, often referred to as the TCP/IP stack. It organizes networking into layers with clear responsibilities: the physical layer handles transmission media (like Ethernet cables), the network layer relies on IP addressing and routers to move packets between networks, the transport layer uses TCP/UDP with port numbers to deliver data to the right application process, and the application layer covers protocols used by end-user software such as web browsers accessing services like Netflix.com.
For learners, the traditional TCP model is often presented with layers 1 through 4 (and sometimes layer 1 split into sublayers in CCNA-focused materials). In that framing, Ethernet and network interface hardware map to layer 1, MAC addressing and switching map to layer 2, and IP addressing and routing map to layer 3.
OSI enters as the competing, historically influential seven-layer framework: Physical, Data Link, Network, Transport, Session, Presentation, and Application. TCP/IP ultimately won adoption, but OSI remains deeply embedded in how network engineers talk about troubleshooting and design. The key difference is that OSI’s extra Session and Presentation layers get folded into TCP/IP’s application layer. Even so, engineers still default to OSI terminology—especially when they say “Layer 7” for application-level problems.
The practical takeaway for CCNA study is that both models matter. OSI is heavily referenced in exams and troubleshooting (“switch is Layer 2,” “router is Layer 3,” “hubs are Layer 1”), while TCP/IP is what real operating systems implement. Memorization tools and layer-by-layer device mapping help turn the abstract models into concrete troubleshooting decisions.
Cornell Notes
TCP/IP and OSI are layered networking models that standardize how devices communicate across different vendors. Early networks were often proprietary, so devices couldn’t reliably exchange data; shared standards made interoperability possible (e.g., a Raspberry Pi and an iPhone exchanging data). TCP/IP is the model most implemented in real systems and is commonly taught with layers that map to Ethernet/physical transmission, MAC switching, IP routing, and TCP/UDP transport plus application protocols. OSI uses seven layers, adding Session and Presentation, but those functions are effectively folded into TCP/IP’s application layer. Even after TCP/IP became dominant, OSI remains the common troubleshooting language—especially “Layer 7” for application issues.
Why did networking models like TCP/IP and OSI become necessary rather than optional?
What role did ARPANET and packet switching play in modern networking?
How do TCP/IP layers map to real networking functions (as taught for CCNA)?
What’s the main conceptual difference between OSI and TCP/IP?
Why do network engineers still talk in OSI terms even though TCP/IP won adoption?
How can you identify common devices by OSI layer (based on the quiz examples)?
Review Questions
- Which OSI layer is most associated with MAC-address-based forwarding, and what device type typically performs that function?
- A troubleshooting symptom is described as “Layer 7.” What kinds of issues are most likely in that category, and why does OSI terminology matter?
- Match these devices to OSI layers: switch, router, hub, repeater. What distinguishes each one’s primary layer responsibility?
Key Points
- 1
Networking interoperability required shared standards after proprietary networks prevented devices from communicating across vendors.
- 2
ARPANET (1969) and packet switching helped establish the core mechanics of sending data across networks.
- 3
TCP/IP is the most widely implemented model and is taught with layers that map to physical transmission, MAC switching, IP routing, and TCP/UDP transport.
- 4
OSI uses seven layers and adds Session and Presentation, but those functions are effectively grouped into TCP/IP’s application layer.
- 5
OSI remains the dominant troubleshooting language in practice, especially when engineers reference “Layer 7.”
- 6
For CCNA preparation, memorizing which devices primarily operate at which OSI layers (switch=Layer 2, router=Layer 3, hub=Layer 1) turns theory into exam-ready recall.