What is the Application Layer and how does it relate to the OSI and TCP/IP models?
The application layer is the topmost layer, closest to the user, where programs exchange data; in TCP/IP it rolls the OSI application, presentation, and session layers into one.
Application Layer:
The layer closest to the end user that is used to exchange data between programs running on the source and destination hosts.
OSI vs TCP/IP Model Mapping:
| OSI Model | TCP/IP Model |
|---|---|
| 7. Application | Application |
| 6. Presentation | Application |
| 5. Session | Application |
| 4. Transport | Transport |
| 3. Network | Internet |
| 2. Data Link | Network Access |
| 1. Physical | Network Access |
Key point: The upper three layers of the OSI model (Application, Presentation, Session) define the functions of the TCP/IP Application layer.
Common Application Layer Protocols: HTTP, FTP, TFTP, IMAP, DNS, SMTP, POP, DHCP
Go deeper:
-
TCP/IP and the OSI Model Explained! — walks through how the layered models line up. -
Application layer (Wikipedia) — how the layer is defined in both TCP/IP and OSI. -
OSI model (Wikipedia) — the seven layers and how TCP/IP folds the top three into one.