OSI Model Concepts

The standard model for networking protocols and distributed applications is the International Standard Organization’s Open System Interconnect (ISO/OSI) model. It defines seven network layers
Short for Open System Interconnection, an ISO standard for worldwide communications that defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Layer 1 – Physical

Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded twisted pairs (UTP)

Layer 2 – Data Link

Data Link layer defines the format of data on the network

Layer 3 – Network

The network layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram. The Internetwork Protocol identifies each host with a 32-bit IP address
Layer 4 – Transport
Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer

Layer 5 – Session

The session protocol defines the format of the data sent over the connections

Layer 6 – Presentation

External Data Representation (XDR) sits at the presentation level. It converts local representation of data to its canonical form and vice versa.

Layer 7 – Application

Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications