Network Fundamentals:

A network consists of some network devices and their interconnection using some media like copper wire or optical fiber or wireless link. The network devices can be classified into the following categories:

End devices:
These are the devices that provides with human interface like PC, Servers, Workstation, Network Printer, Closed Circuit cameras etc.
Intermidiary Devices: These devices are used to connect and control the flow of data, find the optimum route for destination. Most common are Router, Switch, Hub, Wireless access points
Security Devices: These provides security and filtering of data like Firewall

LAN & WAN:
LAN is the network of some private PCs usually located at the same geographic are. WANs are connection of several LANs.

Internet & Intranet:
Internet is the interconnection of a lot of WANs in which data is publicly accessible. Intranet is the interconnection of some WANs of a company that is accessible only by the employees and permitted users.

Protocols:
When we interact with someone we use some rules so that the person at the other end understands. Similarly the network devices when communicating with each other need to abide by some rules. These rules are called Protocols. Protocols specify the structure of the data, how the devices communicate to share different routes, the error and service message provided to the user and the start and termination of a data session. Usually IEEE, IETF, RFCs and other international society defines the protocols so that all over the world the communication is possible.
The important thing is that protocol only provides the rules, not the process by which they are implemented.

Layered Models:
The total network process is divided into several layers. Each layer works independently, so if a network switches from IPv4 to IPv6 only the network layer changes, the rest remains the same.

There are two basic types of models:
TCP/IP model
OSI reference model

OSI model consists of seven layers:
• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical
TCP/IP model has 4 layers:
• Application (Application, Presentation & Session layer of OSI model)
• Transport
• Internet (Network Layer)
• Network Access (Data Link & Physical layer)