Suggest an editImprove this articleRefine the answer for “Why isn't the OSI model used directly in real networks?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)The **OSI model** is not used directly because it is a theoretical scheme of network interaction that was not based on protocols that already existed, unlike TCP/IP, which was a real, working stack from the start. **Key point:** TCP/IP appeared earlier, became the de facto standard, and OSI's protocols (such as CLNP) never gained adoption.Shown above the full answer for quick recall.Answer (EN)Image## **Why the OSI model is not used directly** ### 1. **OSI is a theoretical model, not a working protocol stack** OSI was designed as an *ideal scheme* of network interaction, but it was not based on protocols that already existed. It was a "how it should be" project. Unlike it: - TCP/IP is a *real stack* that was created earlier - TCP/IP was practical and working from the start - its protocols were already in use in real networks before the OSI standards appeared --- ### 2. **The TCP/IP stack appeared earlier and became the de facto standard** While OSI was still under development, TCP/IP was already running on ARPANET and spreading quickly. Protocols: - IP - TCP - UDP - ICMP - DNS - HTTP were already the norm. Hardware vendors and operating systems (UNIX, later Windows) adopted TCP/IP specifically. OSI simply **didn't make it in time**. --- ### 3. **OSI protocols never gained wide adoption** Although OSI tried to offer its own protocols (for example, CLNP), they turned out to be: - complex - slow - incompatible - inefficient - without industry support In the end, they were abandoned. --- ### 4. **OSI separates layers too rigidly and in too much detail** In reality, many protocols do NOT fit neatly into a single layer. For example: - TLS is encryption: is it L6? L7? - SIP is a signaling protocol with L5-L7 functions - Wi-Fi includes both L1 and L2 functions In TCP/IP, this is simplified: - 4 layers instead of 7 - fewer theoretical boundaries - more practical flexibility --- ### 5. **OSI turned out to be too complex to implement** For example: - separating the Presentation (L6) and Session (L5) layers turned out to be **redundant** - vendors did not want to create separate protocols for L5-L6 - TCP/IP merges these layers into the Application Layer, and this is simpler Simplification turned out to be more advantageous and faster to adopt. --- ### 6. **The internet was already running on TCP/IP when OSI appeared** By the time the OSI model was proposed, the internet was already the de facto standard: - universities - government networks - military networks - later, the entire commercial internet All of them used TCP/IP. No one wanted to switch to OSI. --- ### 7. **The cost of implementing OSI was too high** Switching to OSI would have meant: - replacing hardware - rewriting software - changing network architecture This was unrealistic, especially when TCP/IP was already working perfectly well. --- ## **Summary: so what is OSI for?** Although OSI is not used as a protocol stack, it is useful: - for teaching - for systematizing networking technologies - for classifying protocols - for simplifying diagnostics ("the problem is at layer L1/L2/L3") In other words, OSI is an **ideal scheme**, while TCP/IP is a **working tool**. --- ## **Ready answer for an interview** > **OSI is a theoretical 7-layer model created to describe network interaction, but not based on real protocols.** > **By the time it appeared, the TCP/IP stack already existed, was simpler and more efficient, and had become the internet standard.** > **OSI's protocols never gained adoption, and the model turned out to be too complex and redundant for real-world implementation. That is why real networks use TCP/IP, while OSI serves as an educational and analytical model.**For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.