Why isn't the OSI model used directly in real networks?

#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

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#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.

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#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.

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#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

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#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.

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#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.

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#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.

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#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.

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#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.