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====== CCNA - OSI - TCP/IP ======

===== TL;DR =====
**OSI** is a 7-layer reference model that helps you understand and **troubleshoot** networking step by step.

**TCP/IP** is the practical model used on **real networks** and the internet; it has fewer layers because it groups some OSI layers together.

===== Key takeaways =====
  * OSI is a **troubleshooting/learning** model
  * TCP/IP is the **real world** model
  * OSI has **7 layers**
  * TCP/IP has **5 layers** (or 4 layers)

===== OSI model =====
^ Layer ^ Name ^ Description ^
| 7 | Application | The application (i.e. Web browsers, email programs, etc...) |
| 6 | Presentation | Establish data formatting and data translation |
| 5 | Session | Creates the setup, controls the connection and ends the [[https://en.wikipedia.org/wiki/Teardown_(communications)|teardown]] between devices |
| 4 | Transport | The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source host to a destination host from one application to another across a network while maintaining the quality-of-service functions. Transport protocols may be connection-oriented or connectionless. |
| 3 | Network | Transfer packets from one node to another connected in "different networks" |
| 2 | Data Link | Node-to-node data transfer, defines the protocol to establish/terminate a connection between two physically connected devices. Defines protocol for [[https://en.wikipedia.org/wiki/Flow_control_(data)|flow control]] between them |
| 1 | Physical | Transmits raw data between a device and a transmission medium |

===== TCP/IP model =====
^ Layer ^ Name ^ Description ^
| 5 | Application | The services and protocols apps use (i.e. web traffic) |
| 4 | Transport | Protocols like TCP and UDP, plus port numbers |
| 3 | Network | IP Addresses & routing between networks |
| 2 | Data Link | MAC Addresses & local network communication |
| 1 | Physical | Cables, signals, network cards, electrical or wireless transmission |

Data link and Physical layers can be merged into the Network Interface layer in the TCP/IP 4 layers model.