How does OSI differ from TCP/IP?

#Short answer for an interview

OSI is a 7-layer theoretical model created to standardize networks. TCP/IP is a 4-layer practical model that the modern internet is built on. OSI explains how things should work; TCP/IP shows how they actually work**.**

Now, a detailed breakdown.

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#1. Number of layers

OSI layers:

1. Physical
2. Data Link
3. Network
4. Transport
5. Session
6. Presentation
7. Application

TCP/IP layers:

1. Network Access
2. Internet
3. Transport
4. Application

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#2. Theoretical vs. practical model

#OSI - a theoretical model

• created for standardization
• almost never used "as is"
• helps in learning networks
• used for analysis and diagnostics

#TCP/IP - a practical model

• the internet runs on it
• supported by all OSes and devices
• standardized by the IETF
• describes real protocols

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#3. OSI layers 5 and 6 are absent in TCP/IP

TCP/IP has no separate Session and Presentation layers.

Their functions are merged into the Application Layer.

Examples:

• encryption (TLS)
• encoding (UTF-8, MIME)
• session management (cookies, HTTP sessions)

These functions sit between the application and the transport layer, but the TCP/IP model does not separate them into distinct layers.

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#4. Mapping between OSI and TCP/IP layers

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#5. Development: who created it and why

#OSI

• developed by ISO
• goal: to unify networks from different vendors
• focused on standardization

#TCP/IP

• developed by DARPA
• goal: to build a resilient, working network (later the internet)
• focused on practice and operation

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#6. Flexibility and universality

#OSI

Strictly separates layers → easier to explain and teach.

#TCP/IP

Less formal → easier to implement and adapt.

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#7. Set of protocols

#OSI

Protocols existed (CLNP), but never gained wide adoption.

#TCP/IP

The whole model is built on specific protocols, for example:

• IP
• TCP
• UDP
• DNS
• HTTP
• ICMP
• DHCP

This model effectively became the foundation of the internet.

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#8. Real-world use

• TCP/IP - used all the time, everywhere
• OSI - mostly for teaching, network engineering, and describing how layers interact

Still, OSI is more convenient for diagnostics: "Where's the problem? At L1, L2, or L3?"

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#Ready answer for an interview (ideal)

OSI is a seven-layer reference model created to describe network interaction. It is theoretical and is used for teaching and analysis. TCP/IP is a four-layer practical model that the internet is built on. It combines the OSI layers and describes the real protocol stack: HTTP, TCP, IP, Ethernet, and so on. The main differences: the number of layers, practical applicability, and how functions are segmented.

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#Cheat sheet (5 seconds)

• OSI - 7 layers, theoretical.
• TCP/IP - 4 layers, practical.
• OSI = we study it, TCP/IP = we use it.
• TCP/IP has no Session or Presentation.
• TCP/IP describes real protocols (IP, TCP, UDP).